How does one define a function that takes an optional array with an empty array as default?
public void DoSomething(int index, ushort[] array = new ushort[] {}, bool thirdParam = true) results in:
Default parameter value for 'array' must be a compile-time constant.
You can't create compile-time constants of object references.
The only valid compile-time constant you can use is null, so change your code to this:
public void DoSomething(int index, ushort[] array = null, bool thirdParam = true) And inside your method do this:
array = array ?? new ushort[0]; (from comments) From C# 8 onwards you can also use the shorter syntax:
array ??= new ushort[0]; 
null, did you use the second statement from my answer?
array ??= new ushort[0];
??= is available in C# 8If you can make the array the last argument you could also do this:
public void DoSomething(int index, bool wasThirdParam = true, params ushort[] array) The compiler will automatically pass an empty array if it is not specified, and you get the added flexibility to either pass an array as a single argument or put the elements directly as variable length arguments to your method.
I know it's an old question, and whilst this answer doesn't directly solve how to get around the limitations imposed by the compiler, method overloading is an alternative:
public void DoSomething(int index, bool thirdParam = true){ DoSomething(index, new ushort[] {}, thirdParam); } public void DoSomething(int index, ushort[] array, bool thirdParam = true){ ... } 
-foptimize-sibling-calls in g++, but it is C++, I am not sure whether C# compilers can it).