How to initialize generic parameter type T?

Question

Simple question:
If you have a string x, to initialize it you simple do one of the following:

string x = String.Empty;   

or

string x = null; 

What about Generic parameter T?

I've tried doing:

void someMethod<T>(T y) {     T x = new T();       ... } 

Generate error :
Cannot create an instance of the variable type 'T' because it does not have the new() constraint

Answer 1

You have two options:

You can constrain T: you do this by adding: where T : new() to your method. Now you can only use the someMethod with a type that has a parameterless, default constructor (see Constraints on Type Parameters).

Or you use default(T). For a reference type, this will give null. But for example, for an integer value this will give 0 (see default Keyword in Generic Code).

Here is a basic console application that demonstrates the difference:

using System;  namespace Stackoverflow {     class Program     {         public static T SomeNewMethod<T>()             where T : new()         {             return new T();         }          public static T SomeDefaultMethod<T>()             where T : new()         {             return default(T);         }          struct MyStruct { }          class MyClass { }          static void Main(string[] args)         {             RunWithNew();             RunWithDefault();         }          private static void RunWithDefault()         {             MyStruct s = SomeDefaultMethod<MyStruct>();             MyClass c = SomeDefaultMethod<MyClass>();             int i = SomeDefaultMethod<int>();             bool b = SomeDefaultMethod<bool>();              Console.WriteLine("Default");             Output(s, c, i, b);         }          private static void RunWithNew()         {             MyStruct s = SomeNewMethod<MyStruct>();             MyClass c = SomeNewMethod<MyClass>();             int i = SomeNewMethod<int>();             bool b = SomeNewMethod<bool>();              Console.WriteLine("New");             Output(s, c, i, b);         }          private static void Output(MyStruct s, MyClass c, int i, bool b)         {             Console.WriteLine("s: " + s);             Console.WriteLine("c: " + c);             Console.WriteLine("i: " + i);             Console.WriteLine("b: " + b);         }      } } 

It produces the following output:

New s: Stackoverflow.Program+MyStruct c: Stackoverflow.Program+MyClass i: 0 b: False Default s: Stackoverflow.Program+MyStruct c: i: 0 b: False