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Instantiate an object with a runtime-determined type

Asked 2009-06-11T14:09:59.187
83

I'm in a situation where I'd like to instantiate an object of a type that will be determined at runtime. I also need to perform an explicit cast to that type.

Something like this:

static void castTest(myEnum val)
{
    //Call a native function that returns a pointer to a structure
    IntPtr = someNativeFunction(..params..);

    //determine the type of the structure based on the enum value
    Type structType = getTypeFromEnum(val);

    structType myStruct = (structType)Marshal.PtrToStructure(IntPtr, structType);
}

This is obviously not valid code, but I hope it conveys the essence of what I'm trying to do. The method I'm actually working on will have to perform the marshaling operation on ~35 different types. I have several other methods that will need to do something similar with the same set of types. So, I'd like to isolate the type-determining logic from these methods so that I only need to write it once, and so that the methods stay clean and readable.

I must admit to being a total novice at design. Could anyone suggest a good approach to this problem? I suspect there might be an appropriate design pattern that I'm unaware of.

c# .net design-patterns runtime
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1 Answer

19

You can mostly do what you're describing, but since you don't know the type at compile-time, you'll have to keep the instance loosely-typed; check its type at each point you use it, and cast it appropriately then (this will not be necessary with c# 4.0, which supports dynamics):

Type type = CustomGetTypeMethod();
var obj = Activator.CreateInstance(type);

...


if(obj is MyCustomType)
{
    ((MyCustomType)obj).Property1;
}
else if (obj is MyOtherCustomType)
{
    ((MyOtherCustomType)obj).Property2;
}
answered 2009-06-11T14:14:51.017

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