Default method parameters in C#

问题

How can I make a method have default values for parameters?

回答1:

A simple solution is to overload the method:

private void Foo(int length)
{
}

private void Foo()
{
    Foo(20);
}

回答2:

You can only do this in C# 4, which introduced both named arguments and optional parameters:

public void Foo(int x = 10)
{
    Console.WriteLine(x);
}

...
Foo(); // Prints 10

Note that the default value has to be a constant - either a normal compile-time constant (e.g. a literal) or:

The parameterless constructor of a value type
default(T) for some type T

Also note that the default value is embedded in the caller's assembly (assuming you omit the relevant argument) - so if you change the default value without rebuilding the calling code, you'll still see the old value.

This (and other new features in C# 4) are covered in the second edition of C# in Depth. (Chapter 13 in this case.)

回答3:

C# 4.0 allows you to use named and optional arguments:

public void ExampleMethod(
    int required, 
    string optionalstr = "default string",
    int optionalint = 10
)

In previous versions you could simulate default parameters by using method overloading.

回答4:

You simply declare them with the default values - they are called optional parameters:

 public void myMethod(string param1 = "default", int param2 = 3)
 {
 }

This was introduced in C# 4.0 (so you will need to use visual studio 2010).

来源：https://stackoverflow.com/questions/3482528/default-method-parameters-in-c-sharp

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