string source = "/once/upon/a/time/"; int count = 0; foreach (char c in source) if (c == '/') count++; 

Has to be faster than the source.Replace() by itself.

int count = new Regex(Regex.Escape(needle)).Matches(haystack).Count; 

If you want to be able to search for whole strings, and not just characters:

src.Select((c, i) => src.Substring(i)) .Count(sub => sub.StartsWith(target)) 

Read as "for each character in the string, take the rest of the string starting from that character as a substring; count it if it starts with the target string."

LINQ works on all collections, and since strings are just a collection of characters, how about this nice little one-liner:

var count = source.Count(c => c == '/'); 

Make sure you have using System.Linq; at the top of your code file, as .Count is an extension method from that namespace.

I've made some research and found that Richard Watson's solution is fastest in most cases. That's the table with results of every solution in the post (except those use Regex because it throws exceptions while parsing string like "test{test")

 Name | Short/char | Long/char | Short/short| Long/short | Long/long | Inspite | 134| 1853| 95| 1146| 671| LukeH_1 | 346| 4490| N/A| N/A| N/A| LukeH_2 | 152| 1569| 197| 2425| 2171| Bobwienholt | 230| 3269| N/A| N/A| N/A| Richard Watson| 33| 298| 146| 737| 543| StefanosKargas| N/A| N/A| 681| 11884| 12486| 

You can see that in case of finding number of occurences of short substrings (1-5 characters) in short string(10-50 characters) the original algorithm is preferred.

Also, for multicharacter substring you should use the following code (based on Richard Watson's solution)

int count = 0, n = 0; if(substring != "") { while ((n = source.IndexOf(substring, n, StringComparison.InvariantCulture)) != -1) { n += substring.Length; ++count; } } 

string source = "/once/upon/a/time/"; int count = 0; int n = 0; while ((n = source.IndexOf('/', n)) != -1) { n++; count++; } 

On my computer it's about 2 seconds faster than the for-every-character solution for 50 million iterations.

2013 revision:

Change the string to a char[] and iterate through that. Cuts a further second or two off the total time for 50m iterations!

char[] testchars = source.ToCharArray(); foreach (char c in testchars) { if (c == '/') count++; } 

This is quicker still:

char[] testchars = source.ToCharArray(); int length = testchars.Length; for (int n = 0; n < length; n++) { if (testchars[n] == '/') count++; } 

For good measure, iterating from the end of the array to 0 seems to be the fastest, by about 5%.

int length = testchars.Length; for (int n = length-1; n >= 0; n--) { if (testchars[n] == '/') count++; } 

I was wondering why this could be and was Googling around (I recall something about reverse iterating being quicker), and came upon this SO question which annoyingly uses the string to char[] technique already. I think the reversal trick is new in this context, though.

What is the fastest way to iterate through individual characters in a string in C#?

These both only work for single-character search terms...

countOccurences("the", "the answer is the answer"); int countOccurences(string needle, string haystack) { return (haystack.Length - haystack.Replace(needle,"").Length) / needle.Length; } 

may turn out to be better for longer needles...

But there has to be a more elegant way. :)

Edit:

source.Split('/').Length-1 

Regex.Matches(input, Regex.Escape("stringToMatch")).Count 

.NET 8 Span.Count() one-liner

int count = yourString.AsSpan().Count('/'); 

Should be vectorized/hyperfast. Can't beat the readability and maintainability. Also, shouldn't allocate (no garbage).

If anyone wants to benchmark it, feel free to edit this answer to add your benchmark.

https://learn.microsoft.com/en-us/dotnet/api/system.memoryextensions.count?view=net-8.0#system-memoryextensions-count-1(system-readonlyspan((-0))-0)

In C#, a nice String SubString counter is this unexpectedly tricky fellow:

public static int CCount(String haystack, String needle) { return haystack.Split(new[] { needle }, StringSplitOptions.None).Length - 1; } 

private int CountWords(string text, string word) { int count = (text.Length - text.Replace(word, "").Length) / word.Length; return count; } 

Because the original solution, was the fastest for chars, I suppose it will also be for strings. So here is my contribution.

For the context: I was looking for words like 'failed' and 'succeeded' in a log file.

Gr, Ben

string s = "65 fght 6565 4665 hjk"; int count = 0; foreach (Match m in Regex.Matches(s, "65")) count++; 

Well as of .NET 5 (Net core 2.1+ & NetStandard 2.1) we have a new iteration speed king.

"Span<T>" https://learn.microsoft.com/en-us/dotnet/api/system.span-1?view=net-5.0

and String has a built-in member that returns us a Span<Char>

int count = 0; foreach( var c in source.AsSpan()) { if (c == '/') count++; } 

My tests show 62% faster than a straight foreach. I also compared to a for() loop on a Span<T>[i], as well as a few others posted here. Note that the reverse for() iteration on a String seems to run slower now than a straight foreach.

Starting test, 10000000 iterations (base) foreach = 673 ms fastest to slowest foreach Span = 252 ms 62.6% Span [i--] = 282 ms 58.1% Span [i++] = 402 ms 40.3% for [i++] = 454 ms 32.5% for [i--] = 867 ms -28.8% Replace = 1905 ms -183.1% Split = 2109 ms -213.4% Linq.Count = 3797 ms -464.2% 

UPDATE: Dec 2021, Visual Studio 2022, .NET 5 & 6

.NET 5 Starting test, 100000000 iterations set (base) foreach = 7658 ms fastest to slowest foreach Span = 3710 ms 51.6% Span [i--] = 3745 ms 51.1% Span [i++] = 3932 ms 48.7% for [i++] = 4593 ms 40.0% for [i--] = 7042 ms 8.0% (base) foreach = 7658 ms 0.0% Replace = 18641 ms -143.4% Split = 21469 ms -180.3% Linq = 39726 ms -418.8% Regex Compiled = 128422 ms -1,577.0% Regex = 179603 ms -2,245.3% .NET 6 Starting test, 100000000 iterations set (base) foreach = 7343 ms fastest to slowest foreach Span = 2918 ms 60.3% for [i++] = 2945 ms 59.9% Span [i++] = 3105 ms 57.7% Span [i--] = 5076 ms 30.9% (base) foreach = 7343 ms 0.0% for [i--] = 8645 ms -17.7% Replace = 18307 ms -149.3% Split = 21440 ms -192.0% Linq = 39354 ms -435.9% Regex Compiled = 114178 ms -1,454.9% Regex = 186493 ms -2,439.7% 

I added more loops and threw in RegEx so we can see what a disaster it is to use in a lot of iterations. I think the for(++) loop comparison may have been optimized in .NET 6 to use Span internally - since it's almost the same speed as the foreach span.

Code Link

As of .NET 7, we have allocation-free (and highly optimized) Regex APIs. Counting is especially easy and efficient.

 var input = "abcd abcabc ababc"; var result = Regex.Count(input: input, pattern: "abc"); // 4 

When matching dynamic patterns, remember to escape them:

public static int CountOccurences(string input, string pattern) { pattern = Regex.Escape(pattern); // Aww, no way to avoid heap allocations here var result = Regex.Count(input: input, pattern: pattern); return result; } 

And, as a bonus for fixed patterns, .NET 7 introduces analyzers that help convert the regex string to source-generated code. Not only does this avoid the runtime compilation overhead for the regex, but it also provides very readable code that shows how it is implemented. In fact, that code is generally at least as efficient as any alternative you would have written manually.

If your regex call is eligible, the analyzer will give a hint. Simply choose "Convert to 'GeneratedRegexAttribute'" and enjoy the result:

[GeneratedRegex("abc")] private static partial Regex MyRegex(); // Go To Definition to see the generated code 

For anyone wanting a ready to use String extension method,

here is what I use which was based on the best of the posted answers:

public static class StringExtension { /// <summary> Returns the number of occurences of a string within a string, optional comparison allows case and culture control. </summary> public static int Occurrences(this System.String input, string value, StringComparison stringComparisonType = StringComparison.Ordinal) { if (String.IsNullOrEmpty(value)) return 0; int count = 0; int position = 0; while ((position = input.IndexOf(value, position, stringComparisonType)) != -1) { position += value.Length; count += 1; } return count; } /// <summary> Returns the number of occurences of a single character within a string. </summary> public static int Occurrences(this System.String input, char value) { int count = 0; foreach (char c in input) if (c == value) count += 1; return count; } } 

public static int GetNumSubstringOccurrences(string text, string search) { int num = 0; int pos = 0; if (!string.IsNullOrEmpty(text) && !string.IsNullOrEmpty(search)) { while ((pos = text.IndexOf(search, pos)) > -1) { num ++; pos += search.Length; } } return num; } 

I think the easiest way to do this is to use the Regular Expressions. This way you can get the same split count as you could using myVar.Split('x') but in a multiple character setting.

string myVar = "do this to count the number of words in my wording so that I can word it up!"; int count = Regex.Split(myVar, "word").Length; 

I felt that we were lacking certain kinds of sub string counting, like unsafe byte-by-byte comparisons. I put together the original poster's method and any methods I could think of.

These are the string extensions I made.

namespace Example { using System; using System.Text; public static class StringExtensions { public static int CountSubstr(this string str, string substr) { return (str.Length - str.Replace(substr, "").Length) / substr.Length; } public static int CountSubstr(this string str, char substr) { return (str.Length - str.Replace(substr.ToString(), "").Length); } public static int CountSubstr2(this string str, string substr) { int substrlen = substr.Length; int lastIndex = str.IndexOf(substr, 0, StringComparison.Ordinal); int count = 0; while (lastIndex != -1) { ++count; lastIndex = str.IndexOf(substr, lastIndex + substrlen, StringComparison.Ordinal); } return count; } public static int CountSubstr2(this string str, char substr) { int lastIndex = str.IndexOf(substr, 0); int count = 0; while (lastIndex != -1) { ++count; lastIndex = str.IndexOf(substr, lastIndex + 1); } return count; } public static int CountChar(this string str, char substr) { int length = str.Length; int count = 0; for (int i = 0; i < length; ++i) if (str[i] == substr) ++count; return count; } public static int CountChar2(this string str, char substr) { int count = 0; foreach (var c in str) if (c == substr) ++count; return count; } public static unsafe int CountChar3(this string str, char substr) { int length = str.Length; int count = 0; fixed (char* chars = str) { for (int i = 0; i < length; ++i) if (*(chars + i) == substr) ++count; } return count; } public static unsafe int CountChar4(this string str, char substr) { int length = str.Length; int count = 0; fixed (char* chars = str) { for (int i = length - 1; i >= 0; --i) if (*(chars + i) == substr) ++count; } return count; } public static unsafe int CountSubstr3(this string str, string substr) { int length = str.Length; int substrlen = substr.Length; int count = 0; fixed (char* strc = str) { fixed (char* substrc = substr) { int n = 0; for (int i = 0; i < length; ++i) { if (*(strc + i) == *(substrc + n)) { ++n; if (n == substrlen) { ++count; n = 0; } } else n = 0; } } } return count; } public static int CountSubstr3(this string str, char substr) { return CountSubstr3(str, substr.ToString()); } public static unsafe int CountSubstr4(this string str, string substr) { int length = str.Length; int substrLastIndex = substr.Length - 1; int count = 0; fixed (char* strc = str) { fixed (char* substrc = substr) { int n = substrLastIndex; for (int i = length - 1; i >= 0; --i) { if (*(strc + i) == *(substrc + n)) { if (--n == -1) { ++count; n = substrLastIndex; } } else n = substrLastIndex; } } } return count; } public static int CountSubstr4(this string str, char substr) { return CountSubstr4(str, substr.ToString()); } } } 

Followed by the test code...

static void Main() { const char matchA = '_'; const string matchB = "and"; const string matchC = "muchlongerword"; const string testStrA = "_and_d_e_banna_i_o___pfasd__and_d_e_banna_i_o___pfasd_"; const string testStrB = "and sdf and ans andeians andano ip and and sdf and ans andeians andano ip and"; const string testStrC = "muchlongerword amuchlongerworsdfmuchlongerwordsdf jmuchlongerworijv muchlongerword sdmuchlongerword dsmuchlongerword"; const int testSize = 1000000; Console.WriteLine(testStrA.CountSubstr('_')); Console.WriteLine(testStrA.CountSubstr2('_')); Console.WriteLine(testStrA.CountSubstr3('_')); Console.WriteLine(testStrA.CountSubstr4('_')); Console.WriteLine(testStrA.CountChar('_')); Console.WriteLine(testStrA.CountChar2('_')); Console.WriteLine(testStrA.CountChar3('_')); Console.WriteLine(testStrA.CountChar4('_')); Console.WriteLine(testStrB.CountSubstr("and")); Console.WriteLine(testStrB.CountSubstr2("and")); Console.WriteLine(testStrB.CountSubstr3("and")); Console.WriteLine(testStrB.CountSubstr4("and")); Console.WriteLine(testStrC.CountSubstr("muchlongerword")); Console.WriteLine(testStrC.CountSubstr2("muchlongerword")); Console.WriteLine(testStrC.CountSubstr3("muchlongerword")); Console.WriteLine(testStrC.CountSubstr4("muchlongerword")); var timer = new Stopwatch(); timer.Start(); for (int i = 0; i < testSize; ++i) testStrA.CountSubstr(matchA); timer.Stop(); Console.WriteLine("CS1 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrB.CountSubstr(matchB); timer.Stop(); Console.WriteLine("CS1 and: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrC.CountSubstr(matchC); timer.Stop(); Console.WriteLine("CS1 mlw: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountSubstr2(matchA); timer.Stop(); Console.WriteLine("CS2 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrB.CountSubstr2(matchB); timer.Stop(); Console.WriteLine("CS2 and: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrC.CountSubstr2(matchC); timer.Stop(); Console.WriteLine("CS2 mlw: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountSubstr3(matchA); timer.Stop(); Console.WriteLine("CS3 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrB.CountSubstr3(matchB); timer.Stop(); Console.WriteLine("CS3 and: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrC.CountSubstr3(matchC); timer.Stop(); Console.WriteLine("CS3 mlw: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountSubstr4(matchA); timer.Stop(); Console.WriteLine("CS4 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrB.CountSubstr4(matchB); timer.Stop(); Console.WriteLine("CS4 and: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrC.CountSubstr4(matchC); timer.Stop(); Console.WriteLine("CS4 mlw: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountChar(matchA); timer.Stop(); Console.WriteLine("CC1 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountChar2(matchA); timer.Stop(); Console.WriteLine("CC2 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountChar3(matchA); timer.Stop(); Console.WriteLine("CC3 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); timer.Restart(); for (int i = 0; i < testSize; ++i) testStrA.CountChar4(matchA); timer.Stop(); Console.WriteLine("CC4 chr: " + timer.Elapsed.TotalMilliseconds + "ms"); } 

Results: CSX corresponds with CountSubstrX and CCX corresponds with CountCharX. "chr" searches a string for '_', "and" searches a string for "and", and "mlw" searches a string for "muchlongerword"

CS1 chr: 824.123ms CS1 and: 586.1893ms CS1 mlw: 486.5414ms CS2 chr: 127.8941ms CS2 and: 806.3918ms CS2 mlw: 497.318ms CS3 chr: 201.8896ms CS3 and: 124.0675ms CS3 mlw: 212.8341ms CS4 chr: 81.5183ms CS4 and: 92.0615ms CS4 mlw: 116.2197ms CC1 chr: 66.4078ms CC2 chr: 64.0161ms CC3 chr: 65.9013ms CC4 chr: 65.8206ms 

And finally, I had a file with 3.6 million characters. It was "derp adfderdserp dfaerpderp deasderp" repeated 100,000 times. I searched for "derp" inside the file with the above methods 100 times these results.

CS1Derp: 1501.3444ms CS2Derp: 1585.797ms CS3Derp: 376.0937ms CS4Derp: 271.1663ms 

So my 4th method is definitely the winner, but, realistically, if a 3.6 million character file 100 times only took 1586ms as the worse case, then all of this is quite negligible.

By the way, I also scanned for the 'd' char in the 3.6 million character file with 100 times CountSubstr and CountChar methods. Results...

CS1 d : 2606.9513ms CS2 d : 339.7942ms CS3 d : 960.281ms CS4 d : 233.3442ms CC1 d : 302.4122ms CC2 d : 280.7719ms CC3 d : 299.1125ms CC4 d : 292.9365ms 

The original posters method is very bad for single character needles in a large haystack according to this.

Note: All values were updated to Release version output. I accidentally forgot to build on Release mode upon the first time I posted this. Some of my statements have been amended.

string search = "/string"; var occurrences = (regex.Match(search, @"\/")).Count; 

This will count each time the program finds "/s" exactly (case sensitive) and the number of occurrences of this will be stored in the variable "occurrences"

Thought I would throw my extension method into the ring (see comments for more info). I have not done any formal bench marking, but I think it has to be very fast for most scenarios.

EDIT: OK - so this SO question got me to wondering how the performance of our current implementation would stack up against some of the solutions presented here. I decided to do a little bench marking and found that our solution was very much in line with the performance of the solution provided by Richard Watson up until you are doing aggressive searching with large strings (100 Kb +), large substrings (32 Kb +) and many embedded repetitions (10K +). At that point our solution was around 2X to 4X slower. Given this and the fact that we really like the solution presented by Richard Watson, we have refactored our solution accordingly. I just wanted to make this available for anyone that might benefit from it.

Our original solution:

 /// <summary> /// Counts the number of occurrences of the specified substring within /// the current string. /// </summary> /// <param name="s">The current string.</param> /// <param name="substring">The substring we are searching for.</param> /// <param name="aggressiveSearch">Indicates whether or not the algorithm /// should be aggressive in its search behavior (see Remarks). Default /// behavior is non-aggressive.</param> /// <remarks>This algorithm has two search modes - aggressive and /// non-aggressive. When in aggressive search mode (aggressiveSearch = /// true), the algorithm will try to match at every possible starting /// character index within the string. When false, all subsequent /// character indexes within a substring match will not be evaluated. /// For example, if the string was 'abbbc' and we were searching for /// the substring 'bb', then aggressive search would find 2 matches /// with starting indexes of 1 and 2. Non aggressive search would find /// just 1 match with starting index at 1. After the match was made, /// the non aggressive search would attempt to make it's next match /// starting at index 3 instead of 2.</remarks> /// <returns>The count of occurrences of the substring within the string.</returns> public static int CountOccurrences(this string s, string substring, bool aggressiveSearch = false) { // if s or substring is null or empty, substring cannot be found in s if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(substring)) return 0; // if the length of substring is greater than the length of s, // substring cannot be found in s if (substring.Length > s.Length) return 0; var sChars = s.ToCharArray(); var substringChars = substring.ToCharArray(); var count = 0; var sCharsIndex = 0; // substring cannot start in s beyond following index var lastStartIndex = sChars.Length - substringChars.Length; while (sCharsIndex <= lastStartIndex) { if (sChars[sCharsIndex] == substringChars[0]) { // potential match checking var match = true; var offset = 1; while (offset < substringChars.Length) { if (sChars[sCharsIndex + offset] != substringChars[offset]) { match = false; break; } offset++; } if (match) { count++; // if aggressive, just advance to next char in s, otherwise, // skip past the match just found in s sCharsIndex += aggressiveSearch ? 1 : substringChars.Length; } else { // no match found, just move to next char in s sCharsIndex++; } } else { // no match at current index, move along sCharsIndex++; } } return count; } 

And here is our revised solution:

 /// <summary> /// Counts the number of occurrences of the specified substring within /// the current string. /// </summary> /// <param name="s">The current string.</param> /// <param name="substring">The substring we are searching for.</param> /// <param name="aggressiveSearch">Indicates whether or not the algorithm /// should be aggressive in its search behavior (see Remarks). Default /// behavior is non-aggressive.</param> /// <remarks>This algorithm has two search modes - aggressive and /// non-aggressive. When in aggressive search mode (aggressiveSearch = /// true), the algorithm will try to match at every possible starting /// character index within the string. When false, all subsequent /// character indexes within a substring match will not be evaluated. /// For example, if the string was 'abbbc' and we were searching for /// the substring 'bb', then aggressive search would find 2 matches /// with starting indexes of 1 and 2. Non aggressive search would find /// just 1 match with starting index at 1. After the match was made, /// the non aggressive search would attempt to make it's next match /// starting at index 3 instead of 2.</remarks> /// <returns>The count of occurrences of the substring within the string.</returns> public static int CountOccurrences(this string s, string substring, bool aggressiveSearch = false) { // if s or substring is null or empty, substring cannot be found in s if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(substring)) return 0; // if the length of substring is greater than the length of s, // substring cannot be found in s if (substring.Length > s.Length) return 0; int count = 0, n = 0; while ((n = s.IndexOf(substring, n, StringComparison.InvariantCulture)) != -1) { if (aggressiveSearch) n++; else n += substring.Length; count++; } return count; } 

A generic function for occurrences of strings:

public int getNumberOfOccurencies(String inputString, String checkString) { if (checkString.Length > inputString.Length || checkString.Equals("")) { return 0; } int lengthDifference = inputString.Length - checkString.Length; int occurencies = 0; for (int i = 0; i < lengthDifference; i++) { if (inputString.Substring(i, checkString.Length).Equals(checkString)) { occurencies++; i += checkString.Length - 1; } } return occurencies; } 

string source = "/once/upon/a/time/"; int count = 0, n = 0; while ((n = source.IndexOf('/', n) + 1) != 0) count++; 

A variation on Richard Watson's answer, slightly faster with improving efficiency the more times the char occurs in the string, and less code!

Though I must say, without extensively testing every scenario, I did see a very significant speed improvement by using:

int count = 0; for (int n = 0; n < source.Length; n++) if (source[n] == '/') count++; 

 var conditionalStatement = conditionSetting.Value; //order of replace matters, remove == before =, incase of === conditionalStatement = conditionalStatement.Replace("==", "~").Replace("!=", "~").Replace('=', '~').Replace('!', '~').Replace('>', '~').Replace('<', '~').Replace(">=", "~").Replace("<=", "~"); var listOfValidConditions = new List<string>() { "!=", "==", ">", "<", ">=", "<=" }; if (conditionalStatement.Count(x => x == '~') != 1) { result.InvalidFieldList.Add(new KeyFieldData(batch.DECurrentField, "The IsDoubleKeyCondition does not contain a supported conditional statement. Contact System Administrator.")); result.Status = ValidatorStatus.Fail; return result; } 

Needed to do something similar to test conditional statements from a string.

Replaced what i was looking for with a single character and counted the instances of the single character.

Obviously the single character you're using will need to be checked to not exist in the string before this happens to avoid incorrect counts.

String in string:

Find "etc" in " .. JD JD JD JD etc. and etc. JDJDJDJDJDJDJDJD and etc."

var strOrigin = " .. JD JD JD JD etc. and etc. JDJDJDJDJDJDJDJD and etc."; var searchStr = "etc"; int count = (strOrigin.Length - strOrigin.Replace(searchStr, "").Length)/searchStr.Length. 

Check performance before discarding this one as unsound/clumsy...

My initial take gave me something like:

public static int CountOccurrences(string original, string substring) { if (string.IsNullOrEmpty(substring)) return 0; if (substring.Length == 1) return CountOccurrences(original, substring[0]); if (string.IsNullOrEmpty(original) || substring.Length > original.Length) return 0; int substringCount = 0; for (int charIndex = 0; charIndex < original.Length; charIndex++) { for (int subCharIndex = 0, secondaryCharIndex = charIndex; subCharIndex < substring.Length && secondaryCharIndex < original.Length; subCharIndex++, secondaryCharIndex++) { if (substring[subCharIndex] != original[secondaryCharIndex]) goto continueOuter; } if (charIndex + substring.Length > original.Length) break; charIndex += substring.Length - 1; substringCount++; continueOuter: ; } return substringCount; } public static int CountOccurrences(string original, char @char) { if (string.IsNullOrEmpty(original)) return 0; int substringCount = 0; for (int charIndex = 0; charIndex < original.Length; charIndex++) if (@char == original[charIndex]) substringCount++; return substringCount; } 

The needle in a haystack approach using replace and division yields 21+ seconds whereas this takes about 15.2.

Edit after adding a bit which would add substring.Length - 1 to the charIndex (like it should), it's at 11.6 seconds.

Edit 2: I used a string which had 26 two-character strings, here are the times updated to the same sample texts:

Needle in a haystack (OP's version): 7.8 Seconds

Suggested mechanism: 4.6 seconds.

Edit 3: Adding the single character corner-case, it went to 1.2 seconds.

Edit 4: For context: 50 million iterations were used.

Split (may) wins over IndexOf (for strings).

The benchmark above seems to indicate that Richard Watson is the fastest for string which is wrong (maybe the difference comes from our test data but it seems strange anyway for the reasons below).

If we look a bit deeper in the implementation of these methods in .NET (for Luke H, Richard Watson methods),

• IndexOf is culture depending, it will try to retrieve/create ReadOnlySpan, check if it has to ignore case etc.. and then finally do the unsafe / native call.
• Split is able to handle several separators and has some StringSplitOptions and has to create the string[] array and fill it with the split result (so do some substring). Depending on the number of string occurrence Split may be faster than IndexOf.

By the way, I made a simplified version of IndexOf (which could be faster if I used pointer and unsafe but unchecked should be ok for most) which is faster by at least a 4 order of magnitude.

Speed is not the only criteria

While Split may be faster, it will use more memory (and put more pressure on garbage collector), especially if there is a lot of needles. The below implementation tries to minimize memory allocation.

Benchmark (source on GitHub)

Done by searching either a common word (the) or a small sentence in Shakespeare Richard III.

public int GetOccurrences(string input, string needle) { int count = 0; unchecked { if (string.IsNullOrEmpty(input) || string.IsNullOrEmpty(needle)) { return 0; } for (var i = 0; i < input.Length - needle.Length + 1; i++) { var c = input[i]; if (c == needle[0]) { for (var index = 0; index < needle.Length; index++) { c = input[i + index]; var n = needle[index]; if (c != n) { break; } else if (index == needle.Length - 1) { count++; } } } } } return count; } 

string Name = "Very good nice one is very good but is very good nice one this is called the term"; bool valid=true; int count = 0; int k=0; int m = 0; while (valid) { k = Name.Substring(m,Name.Length-m).IndexOf("good"); if (k != -1) { count++; m = m + k + 4; } else valid = false; } Console.WriteLine(count + " Times accures"); 

If you check out this webpage, 15 different ways of doing this are benchmarked, including using parallel loops.

The fastest way appears to be using either a single threaded for-loop (if you have .Net version < 4.0) or a parallel.for loop (if using .Net > 4.0 with thousands of checks).

Assuming "ss" is your Search String, "ch" is your character array (if you have more than one char you're looking for), here's the basic gist of the code that had the fastest run time single threaded:

for (int x = 0; x < ss.Length; x++) { for (int y = 0; y < ch.Length; y++) { for (int a = 0; a < ss[x].Length; a++ ) { if (ss[x][a] == ch[y]) //it's found. DO what you need to here. } } } 

The benchmark source code is provided too so you can run your own tests.