generate 2d barcode vb.net Part II Designing Types in Visual C#

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Part II Designing Types
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as Nullable<Nullable<T>> . Nullable types are discussed in 19, Nullable Value Types . Here is an example class that constrains its type parameter by using the struct constraint:
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internal sealed class PrimaryConstraintOfStruct<T> where T : struct { public static T Factory() { // Allowed because all value types implicitly // 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 value type, and all value types implicitly have a public, parameterless constructor . If T were unconstrained, constrained to a reference type, or constrained to class, the above code would not compile because some reference types do not have public, parameterless constructors .
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A type parameter can specify zero or more secondary constraints where a secondary constraint represents an interface type . When specifying an interface type constraint, you are promising the compiler that a specified type argument will be a type that implements the interface . And since you can specify multiple interface constraints, the type argument must specify a type that implements all of the interface constraints (and all of the primary constraints too, if specified) . 13 discusses interface constraints in detail . There is another kind of secondary constraint called a type parameter constraint (sometimes referred to as a naked type constraint) . This kind of constraint is used much less often than an interface constraint . It allows a generic type or method to indicate that there must be a relationship between specified type arguments . A type parameter can have zero or more type constraints applied to it . Here is a generic method that demonstrates the use of a type parameter constraint:
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private static List<TBase> ConvertIList<T, TBase>(IList<T> list) where T : TBase { List<TBase> baseList = new List<TBase>(list.Count); for (Int32 index = 0; index < list.Count; index++) { baseList.Add(list[index]); } return baseList; }
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The ConvertIList method specifies two type parameters in which the T parameter is constrained by the TBase type parameter . This means that whatever type argument is specified for T, the type argument must be compatible with whatever type argument is specified for TBase . Here is a method showing some legal and illegal calls to ConvertIList:
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private static void CallingConvertIList() { // Construct and initialize a List<String> (which implements IList<String>) IList<String> ls = new List<String>(); ls.Add("A String"); // Convert the IList<String> to an IList<Object> IList<Object> lo = ConvertIList<String, Object>(ls); // Convert the IList<String> to an IList<IComparable> IList<IComparable> lc = ConvertIList<String, IComparable>(ls); // Convert the IList<String> to an IList<IComparable<String>> IList<IComparable<String>> lcs = ConvertIList<String, IComparable<String>>(ls); // Convert the IList<String> to an IList<String> IList<String> ls2 = ConvertIList<String, String>(ls); // Convert the IList<String> to an IList<Exception> IList<Exception> le = ConvertIList<String, Exception>(ls);// Error }
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In the first call to ConvertIList, the compiler ensures that String is compatible with Object . Since String is derived from Object, the first call adheres to the type parameter constraint . In the second call to ConvertIList, the compiler ensures that String is compatible with IComparable . Since String implements the IComparable interface, the second call adheres to the type parameter constraint . In the third call to ConvertIList, the compiler ensures that String is compatible with IComparable<String> . Since String implements the IComparable<String> interface, the third call adheres to the type parameter constraint . In the fourth call to ConvertIList, the compiler knows that String is compatible with itself . In the fifth call to ConvertIList, the compiler ensures that String is compatible with Exception . Since String is not compatible with Exception, the fifth call doesn t adhere to the type parameter constraint, and the compiler issues the following message: "error
CS0311: The type 'string' cannot be used as type parameter 'T' in the generic type or method 'Program.ConvertIList<T,TBase>(System.Collections. Generic.IList<T>)'. There is no implicit reference conversion from 'string' to 'System.Exception'."
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