google didnt help me such as i wanted so im writing post here. i have Unicode string in C# and C function (in dll) which want the char(ANSI)*. i try to do
string tmp = "ala ma kota"; unsafe { fixed (byte* data = &tmp.ToCharArray()[0]) { some_function(data); } } but i cannot convert directly without encoding. I try to use Encode class but without any effects. I know that, the some_function needs to be called with char pointer. Where it points to array of byte.
You don't need to do this explicitly yourself.
Instead, declare the C method using P/Invoke to accept a parameter of type string.
Then add to the P/Invoke declaration:
[DllImport("YourDllName.dll", CharSet=CharSet.Ansi)] The marshaller will then convert the string to ANSI for you.
Note that I'm assuming that the string is being passed TO the called function, so the parameter is NOT a pointer being used to return a new string FROM the called function.
See here for more details: http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.charset.aspx
In fact, the default is CharSet.Ansi so you might only need to declare the parameter as string instead of byte* (or whatever you are using just now).
[MarshalAs(UnmanagedType.LPStr)] declared inline on the parameter since the default string marshaling is UnmanagedType.BStr.
You should use the System.Text namespace:
string tmp = "ala ma kota"; unsafe { fixed (byte* data = &System.Text.Encoding.ASCII.GetBytes(tmp)[0]) { some_function(data); } } 
Encode class will get the bytes in a particular format.ASCII.GetBytes()