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Generic Methods and Type Inference
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For many developers, the C# generic syntax can be confusing with all of its less-than and greater-than signs . To help improve code creation, readability, and maintainability, the C# compiler offers type inference when calling a generic method . Type inference means that the compiler attempts to determine (or infer) the type to use automatically when calling a generic method . Here is some code that demonstrates type inference:
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private static void CallingSwapUsingInference() { Int32 n1 = 1, n2 = 2; Swap(ref n1, ref n2);// Calls Swap<Int32> String s1 = "Aidan"; Object s2 = "Grant"; Swap(ref s1, ref s2);// Error, type can't be inferred }
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The where clause will be explained in the Verifiability and Constraints section later in this chapter .
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In this code, notice that the calls to Swap do not specify type arguments in less-than/greaterthan signs . In the first call to Swap, the C# compiler was able to infer that n1 and n2 are Int32s, and therefore, it should call Swap by using an Int32 type argument . When performing type inference, C# uses the variable s data type, not the actual type of the object referred to by the variable . So in the second call to Swap, C# sees that s1 is a String and s2 is an Object (even though it happens to refer to a String) . Since s1 and s2 are variables of different data types, the compiler can t accurately infer the type to use for Swap s type argument, and it issues the following message: "error CS0411: The type arguments
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for method 'Program.Swap<T>(ref T, ref T)' cannot be inferred from the usage. Try specifying the type arguments explicitly."
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A type can define multiple methods with one of its methods taking a specific data type and another taking a generic type parameter, as in the following example:
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private static void Display(String s) { Console.WriteLine(s); } private static void Display<T>(T o) { Display(o.ToString()); // Calls Display(String) }
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Here are some ways to call the Display method:
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Display("Jeff"); Display(123); Display<String>("Aidan"); // Calls Display(String) // Calls Display<T>(T) // Calls Display<T>(T)
In the first call, the compiler could actually call either the Display method that takes a String or the generic Display method (replacing T with String) . However, the C# compiler always prefers a more explicit match over a generic match, and therefore, it generates a call to the non-generic Display method that takes a String . For the second call, the compiler can t call the non-generic Display method that takes a String, so it must call the generic Display method . By the way, it is fortunate that the compiler always prefers the more explicit match; if the compiler had preferred the generic method, because the generic Display method calls Display again (but with a String returned by ToString), there would have been infinite recursion . The third call to Display specifies a generic type argument, String . This tells the compiler not to try to infer type arguments but instead to use the type arguments that I explicitly specified . In this case, the compiler also assumes that I must really want to call the generic Display method, so the generic Display will be called . Internally, the generic Display method will call ToString on the passed-in string, which results in a string that is then passed to the non-generic Display method .
Part II Designing Types
generics and Other Members
In C#, properties, indexers, events, operator methods, constructors, and finalizers cannot themselves have type parameters . However, they can be defined within a generic type, and the code in these members can use the type s type parameters . C# doesn t allow these members to specify their own generic type parameters because Microsoft s C# team believes that developers would rarely have a need to use these members as generic . Furthermore, the cost of adding generic support to these members would be quite high in terms of designing adequate syntax into the language . For example, when you use a + operator in code, the compiler could call an operator overload method . There is no way to indicate any type arguments in your code along with the + operator .
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