Type 的 C# 通用约束是可转换的

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 ;
}

Answer 1

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 ;
}

Answer 2

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()

Answer 3

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)