C# generics and casting

Question

I've come across a function in our code base throwing an error:

public static T InternalData<T>()
{
    return (T)"100";
}

Obviously I've simplified the code and added the "100" as a literal string value. T is of type int .

It throws a:

System.InvalidCastException: Specified cast is not valid.

It seems that you can't implicitly convert a string to int in C#, how can I fix this code so that it can handle converting any generic type?

The actual code would look something like this:

public static T InternalData<T>()
{
    return (T) something (not sure of type or data);
}

Answer 1

Try:

public static T InternalData<T>(object data)
{
     return (T) Convert.ChangeType(data, typeof(T));
}

This works for types that implement the IConvertible interface (which Int32 and String does).

Answer 2

One possibility would be to use

return (T)Convert.ChangeType(yourValue, typeof(T));

Please note that this will throw an exception, if yourValue isn't an instance of a type that implements IConvertible . It will also throw an exception if the value itself can't be converted, for example if you have "abc" instead of "100".

Answer 3

Use Convert.ChangeType .

public static T InternalData<T>()
{
    return (T)Convert.ChangeType("100", typeof (T));
}

It'll still throw an error if the values can't be converted, but it will not try to do a direct cast. It can convert strings to ints ok.

Answer 4

Do not confuse casting and converting! If the true type of an expression is not known to the compiler, eg because it is typed as object , and you know its inherent type, you can tell the C# compiler, "Hey, I know its a int , so please treat it as a int ".

 (int)expression

In your case the expression is a string expression that cannot be casted to int , simply beacuase it is not an int . However, you can convert the string to int if it represents a valid integer. Moreover the result type of your conversion is unknown because its generic. Use (T)Convert.ChangeType(...) as others have already suggested.