How to make inline functions in C#

Yes, C# supports that. There are several syntaxes available.

• Anonymous methods were added in C# 2.0: Func<int, int, int> add = delegate(int x, int y) { return x + y; }; Action<int> print = delegate(int x) { Console.WriteLine(x); } Action<int> helloWorld = delegate // parameters can be elided if ignored { Console.WriteLine("Hello world!"); }
• Lambdas are new in C# 3.0 and come in two flavours.
  • Expression lambdas: Func<int, int, int> add = (int x, int y) => x + y; // or... Func<int, int, int> add = (x, y) => x + y; // types are inferred by the compiler
  • Statement lambdas: Action<int> print = (int x) => { Console.WriteLine(x); }; Action<int> print = x => { Console.WriteLine(x); }; // inferred types Func<int, int, int> add = (x, y) => { return x + y; };
• Local functions have been introduced with C# 7.0: int add(int x, int y) => x + y; void print(int x) { Console.WriteLine(x); }

There are basically two different types for these: Func and Action. Funcs return values but Actions don’t. The last type parameter of a Func is the return type; all the others are the parameter types.

There are similar types with different names, but the syntax for declaring them inline is the same. An example of this is Comparison<T>, which is roughly equivalent to Func<T, T, int>.

Func<string, string, int> compare1 = (l,r) => 1;
Comparison<string> compare2 = (l, r) => 1;
Comparison<string> compare3 = compare1; // this one only works from C# 4.0 onwards

These can be invoked directly as if they were regular methods:

int x = add(23, 17); // x == 40
print(x); // outputs 40
helloWorld(x); // helloWorld has one int parameter declared: Action<int>
               // even though it does not make any use of it.