C# generic constraint for Type to be castable

Question

Is there a way with C# generics to limit a type T to be castable from another type?

Example:
Lets say I am saving information in the registry as a string, and when I restore the information I would like to have a function that looks something like that:

static T GetObjectFromRegistry<T>(string regPath) where T castable from string 
{
    string regValue = //Getting the registry value...
    T objectValue = (T)regValue;
    return objectValue ;
}

There is no such constraints in .NET – Sergey Berezovskiy Jul 24 '13 at 06:48 — Sergey Berezovskiy, Jul 24 '13 at 06:48

Sergey Berezovskiy · Accepted Answer · 2013-07-24T07:05:35.957

There is no such type of constraints in .NET. There is only six types of constraints available (see Constraints on Type Parameters):

where T: struct type argument must be a value type
where T: class type argument must be a reference type
where T: new() type argument must have a public parameterless constructor
where T: <base class name> type argument must be or derive from the specified base class
where T: <interface name> type argument must be or implement the specified interface
where T: U type argument supplied for T must be or derive from the argument supplied for U

If you want to cast string to your type, you can do casting to object first. But you can't put constraint on type parameter to make sure this casting can occur:

static T GetObjectFromRegistry<T>(string regPath)
{
    string regValue = //Getting the regisstry value...
    T objectValue = (T)(object)regValue;
    return objectValue ;
}

Another option - create interface:

public interface IInitializable
{
    void InitFrom(string s);
}

And put it as constraint:

static T GetObjectFromRegistry<T>(string regPath) 
  where T: IInitializable, new()
{
    string regValue = //Getting the regisstry value...   
    T objectValue = new T();
    objectValue.InitFrom(regValue);
    return objectValue ;
}

score 0 · Answer 2 · edited May 23 '17 at 11:56

0

Types are determined during compilation. You can't change the types during runtime. It is possible to cast object to its base or child class

Ref -

Difference between object a = new Dog() vs Dog a = new Dog()

edited May 23 '17 at 11:56

Community

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answered Jul 24 '13 at 06:49

om471987

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I realize that Types are determined at compilation time. but at compilation time you can also now the type reference passed to the function. (in the same manner that. string a = "abc", a[0] will work while (a as object)[0] won't ) So I don't really see how this answers the question... – Avi Turner Jul 24 '13 at 07:05

score 0 · Answer 3 · answered Jul 24 '13 at 07:05

0

Constraints spell out like "the type of T must either be of type U or inherit type U", so the constraint you are looking for isn't doable.

everything is "castable" to String anyway, through .ToString() (YMMV)

answered Jul 24 '13 at 07:05

Alex

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**YMMV explained:** `.ToString()` is more or less an inbuilt debugging-feature. "[It converts an object to its string representation so that it is suitable for display.](https://msdn.microsoft.com/en-us/library/system.object.tostring.aspX)" - MSDN. I strongly advise against (ab-)using it for casting. – FooBarTheLittle Nov 04 '15 at 09:34

C# generic constraint for Type to be castable

3 Answers3