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The DefaultParameterValue and Optional Attributes
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It would be best if this concept of default and optional arguments was not C#-specific . Specifically, we want programmers to define a method indicating which parameters are optional and what their default value should be in some programming language and then give programmers working in other programming languages the ability to call them . For this to work, the compiler of choice must allow the caller to omit some arguments and have a way of determining what those arguments default values should be . In C#, when you give a parameter a default value, the compiler internally applies the System.Runtime.InteropServices.OptionalAttribute custom attribute to the parameter, and this attribute is persisted in the resulting file s metadata . In addition, the compiler applies System.Runtime.InteropServices.DefaultParameterValueAttribute to the parameter and persists this attribute in the resulting file s metadata . Then, DefaultParameterValueAttribute s constructor is passed the constant value that you specified in your source code . Now, when a compiler sees that you have code calling a method that is missing some arguments, the compiler can ensure that you ve omitted optional arguments, grab their default values out of metadata, and embed the values in the call for you automatically .
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9 Parameters
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Implicitly Typed Local Variables
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C# supports the ability to infer the type of a method s local variable from the type of expression that is used to initialize it . Here is some sample code demonstrating the use of this feature:
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private static void ImplicitlyTypedLocalVariables() { var name = "Jeff"; ShowVariableType(name); // Displays: System.String // var n = null; var x = (Exception)null; ShowVariableType(x); // Error // OK, but not much value // Displays: System.Exception
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var numbers = new Int32[] { 1, 2, 3, 4 }; ShowVariableType(numbers); // Displays: System.Int32[] // Less typing for complex types var collection = new Dictionary<String, Single>() { { ".NET", 4.0f } }; // Displays: System.Collections.Generic.Dictionary`2[System.String,System.Single] ShowVariableType(collection); foreach (var item in collection) { // Displays: System.Collections.Generic.KeyValuePair`2[System.String,System.Single] ShowVariableType(item); } } private static void ShowVariableType<T>(T t) { Console.WriteLine(typeof(T)); }
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The first line of code inside the ImplicitlyTypedLocalVariables method is introducing a new local variable using the C# var token . To determine the type of the name variable, the compiler looks at the type of the expression on the right side of the assignment operator (=) . Since "Jeff" is a string, the compiler infers that name s type must be String . To prove that the compiler is inferring the type correctly, I wrote the ShowVariableType method . This generic method infers the type of its argument, and then it shows the type that it inferred on the console . I added what ShowVariableType displayed as comments inside the ImplicitlyTypedLocalVariables method for easy reading . The second assignment (commented out) inside the ImplicitlyTypedLocalVariables method would produce a compiler error ("error CS0815: Cannot assign <null> to an implicitly-typed local variable") because null is implicitly castable to any reference type or nullable value type; therefore, the compiler cannot infer a distinct type for it . However, on the third assignment, I show that it is possible to initialize an implicitly typed local variable with null if you explicitly specify a type (Exception, in my example) . While this is possible, it is not that useful because you could also write Exception x = null; to get the same result .
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In the fourth assignment, you see some real value of using C# s implicitly typed local variable feature . Without this feature, you d have to specify Dictionary<String, Single> on both sides of the assignment operator . Not only is this a lot of typing, but if you ever decide to change the collection type or any of the generic parameter types, then you would have to modify your code on both sides of the assignment operator, too . In the foreach loop, I also use var to have the compiler automatically infer the type of the elements inside the collection . This demonstrates that it is possible and quite useful to use var with foreach, using, and for statements . It can also be useful when experimenting with code . For example, you initialize an implicitly typed local variable from the return type of a method, and as you develop your method, you might decide to change its return type . If you do this, the compiler will automatically figure out that the return type has changed and automatically change the type of the variable! This is great, but of course, other code in the method that uses that variable may no longer compile if the code accesses members using the variable assuming that it was the old type . In Microsoft Visual Studio, you can hold the mouse cursor over var in your source code and the editor will display a tooltip showing you the type that the compiler infers from the expression . C# s implicitly typed local variable feature must be used when working with anonymous types within a method; see 10, Properties, for more details . You cannot declare a method s parameter type using var . The reason for this should be obvious to you since the compiler would have to infer the parameter s type from the argument being passed at a callsite and there could be no call sites or many call sites . In addition, you cannot declare a type s field using var . There are many reasons why C# has this restriction . One reason is that fields can be accessed by several methods and the C# team feels that this contract (the type of the variable) should be stated explicitly . Another reason is that allowing this would permit an anonymous type (discussed in 10) to leak outside of a single method . Important Do not confuse dynamic and var . Declaring a local variable using var is just a
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syntactical shortcut that has the compiler infer the specific data type from an expression . The var keyword can be used only for declaring local variables inside a method while the dynamic keyword can be used for local variables, fields, and arguments . You cannot cast an expression to var, but you can cast an expression to dynamic . You must explicitly initialize a variable declared using var while you do not have to initialize a variable declared with dynamic . For more information about C# s dynamic type, see the The dynamic Primitive Type section in 5 .
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