vb.net qr code open source Constructor Constraints in .NET framework

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Constructor Constraints
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A type parameter can specify zero constructor constraints or one constructor constraint . When specifying a constructor constraint, you are promising the compiler that a specified type argument will be a non-abstract type that implements a public, parameterless constructor . Note that the C# compiler considers it an error to specify a constructor constraint with the struct constraint because it is redundant; all value types implicitly offer a public, parameterless constructor . Here is an example class that constrains its type parameter by using the constructor constraint:
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Part II Designing Types
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internal sealed class ConstructorConstraint<T> where T : new() { public static T Factory() { // Allowed because all value types implicitly // have a public, parameterless constructor and because // the constraint requires that any specified reference // type also have a public, parameterless constructor return new T(); } }
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In this example, newing up a T is legal because T is known to be a type that has a public, parameterless constructor . This is certainly true of all value types, and the constructor constraint requires that it be true of any reference type specified as a type argument . Sometimes, developers would like to declare a type parameter by using a constructor constraint whereby the constructor takes various parameters itself . As of now, the CLR (and therefore the C# compiler) supports only parameterless constructors . Microsoft feels that this will be good enough for almost all scenarios, and I agree .
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Other Verifiability Issues
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In the remainder of this section, I d like to point out a few other code constructs that have unexpected behavior when used with generics due to verifiability issues and how constraints can be used to make the code verifiable again .
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Casting a Generic Type Variable
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Casting a generic type variable to another type is illegal unless you are casting to a type compatible with a constraint:
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private static void CastingAGenericTypeVariable1<T>(T obj) { Int32 x = (Int32) obj; // Error String s = (String) obj; // Error }
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The compiler issues an error on both lines above because T could be any type, and there is no guarantee that the casts will succeed . You can modify this code to get it to compile by casting to Object first:
private static void CastingAGenericTypeVariable2<T>(T obj) { Int32 x = (Int32) (Object) obj; // No error String s = (String) (Object) obj; // No error }
While this code will now compile, it is still possible for the CLR to throw an InvalidCastException at runtime .
12 Generics
If you are trying to cast to a reference type, you can also use the C# as operator . Here is code modified to use the as operator with String (since Int32 is a value type):
private static void CastingAGenericTypeVariable3<T>(T obj) { String s = obj as String; // No error }
Setting a Generic Type Variable to a Default Value
Setting a generic type variable to null is illegal unless the generic type is constrained to a reference type .
private static void SettingAGenericTypeVariableToNull<T>() { T temp = null; // CS0403 Cannot convert null to type parameter 'T' because it could // be a non-nullable value type. Consider using 'default(T)' instead }
Since T is unconstrained, it could be a value type, and setting a variable of a value type to null is not possible . If T were constrained to a reference type, setting temp to null would compile and run just fine . Microsoft s C# team felt that it would be useful to give developers the ability to set a variable to a default value . So the C# compiler allows you to use the default keyword to accomplish this:
private static void SettingAGenericTypeVariableToDefaultValue<T>() { T temp = default(T); // OK }
The use of the default keyword above tells the C# compiler and the CLR s JIT compiler to produce code to set temp to null if T is a reference type and to set temp to all-bits-zero if T is a value type .
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