What is an input parameter with equality (assignment operator)?

Note that this is not equality, it is the assignment operator.

This is a parameter with default value ( default ). If no argument is passed to fill it, this value is used in that variable. It assigns a value to the variable in the absence of the argument that should assign a value to it during the method call.

The most correct name for the feature is optional arguments , since it allows some arguments not to be passed and the method assumes a value.

Can only be used in the last parameters, can not have a parameter without a default value after it has a default value.

This is an interesting feature because you avoid having to create methods with different signatures to achieve the same goal and / or have to do some algorithm to solve the lack of value.

See What is the difference between parameter and argument? .

Note that if you pass null , the value of the variable will be null, you can not pass argument to assume this value. Can be viewed at:

public static void Main() => WriteLine(teste(null));
public static string teste(string x = "xxx") => x;

See running on .NET Fiddle . Also I put it in Github for future reference .