[C # delicious] default parameters in C #4.0

Document directory

• Before C #4.0
• After 4.0:
• The story behind
• References:

In C #4.0, what attracts me first is the default parameter. When I use this feature very smoothly in Python, C # can only write a large number of overload functions, and then call one function, and finally call the one with the most parameters.

Before C #4.0

Before 4.0, When I want a function parameter to have a default value, it must be written as follows:

static void OldFun(string param1){    OldFun(param1, "default param2");}static void OldFun(string param1, string param2){    OldFun(param1, param2, "default param3");}static void OldFun(string param1, string param2, string param3){    var content = string.Format("{0},{1},{2}",param1,param2,param3);    Console.WriteLine(content);}

I am so miserable that I cannot understand why Microsoft does not provide a default parameter. I envy C ++ programmers.

After 4.0:

In C #4.0, Microsoft finally added this feature.

The new Code is much simpler.

static void Main(string[] args){    OldFun("GreenerycnDemo");     OldFun("GreenerycnDemo", "abc");     OldFun("greenerycn", "cnblogs", "com");}static void OldFun( string param1,                     string param2 = "default param2",                     string param3 = "default param3"){    var content = string.Format("{0},{1},{2}", param1, param2, param3);    Console.WriteLine(content);}

After execution:

 

Note: parameters with default values must be placed at the end of the parameter list, that is, the following method cannot be used; otherwise, the compilation will fail.

static void OldFun(string param2 = "default param2",                    string param1,                    string param3 = "default param3"){    var content = string.Format("{0},{1},{2}",param1,param2,param3);    Console.WriteLine(content);}

Compilation failed:

The story behind

What did Microsoft do with such a convenient feature? Let's take a look at it with reflector:

private static void OldFun(string param1,     [Optional, DefaultParameterValue("default param2")] string param2,     [Optional, DefaultParameterValue("default param3")] string param3){    Console.WriteLine(string.Format("{0},{1},{2}", param1, param2, param3));}

Haha, we added the optional and defaultparametervalue attributes to the parameter signature.

These two attributes are in the namespace system. runtime. interopservices:

• Optionalattribute: this parameter is optional.
• Defaultparametervalueattribute: the default value of this attribute setting parameter.

According to some materials: According to this principle, the optional parameters mentioned above must be followed by the code to specify these two attributes, so that the compilation will be okay. However, I cannot do it myself. An error will still be reported during the call.

 

How can this problem be solved? I will tell you in the next article.

 

References:

• New features in C # 4.0.doc
• C #4.0-named and optional parameters-behind the scenes
• C #4.0 features: named & optional parameters (optional and named parameters)