C＃泛型类：如何限制成员函数与特定类型一起使用？

Question

I have a generic class CGeometryCalibration2D<T> which can be used with numeric types like int , double , float , and my custom type CLocation .
(less important) question 1: how can I restrict T to one of these types?

Inside this class, there is a function

double InterpolateReverse (T i_Value);

which should be used only with CLocation as T .

I don't know any way to apply such a restriction.
I already tried extension methods, which would do exactly this restriction

double InterpolateReverse (this CGeometricCalibration2D<CLocation>, CLocation i_Value);

but they don't allow access to private members. I could work around this limitation by using Reflection, but that's not the nicest way.

What can I do here?
Should I maybe find a completely different approach?

The only remaining idea I have is overloading the generic class by concrete implmentations and adding the function there, like

CGeometricCalibration2D_CLocation : CGeometricCalibration2D<CLocation>
{
  double InterpolateReverse (CLocation i_Value);
}

but then I need to have an object of the concrete type CGeometricCalibration2D_CLocation in order to execute InterpolateReverse ()

Answer 1

I have a generic class CGeometryCalibration2D<T> which can be used with numeric types like int, double, float, and my custom type CLocation.

That doesn't sound like a generic type.

how can I restrict T to one of these types.

You can't, there is no constraint that encompasses those types.

Inside this class there is a function (...) that should only be used with CLocation as T .

Easy, don't make the method generic because it isn't. Simply write a single overlord with a CLocation argument.

Remember, generic means generic . A generic class or method should be able to work with an infinite set of generic type parameters. When the valid types are limited chances are that you are doing something wrong.

Answer 2

You can achieve it by defining additional type parameter on that method, and add type constraint. In order to achieve this, you'll need to create ICLocation interface

public class CGeometryCalibration2D<T> where T: struct
{
    public double InterpolateReverse<V>(V i_Value) where V: struct, ICLocation
    {
        return default(double);
    }
}

public interface ICLocation { }

public struct CLocation : ICLocation { }

Example of usage:

var geoInt = new CGeometryCalibration2D<int>();
geoInt.InterpolateReverse(12); //Compile time error
var loc = new CLocation();
geoInt.InterpolateReverse(loc);

Answer 3

You cannot put a value type constraint on generic parameters, as documented here .

What you could do is create overloads of your function instead of using generics, eg

double InterpolateReverse (int i_Value);
double InterpolateReverse (double i_Value);
double InterpolateReverse (float i_Value);

Answer 4

For the first question, you cannot use multiple types for one generic parameter. Think about it. How will the compiler decide the compile-time type of the parameter in that case? You should use overloaded methods for each of the types that you want to support and simply remove that generic parameter.

This should solve your second problem as well.

Answer 5

Thank you all for your answers and comments!
After thinking about them, I decided to mix generics with inheritance, which hopefully is the cleanest solution to my problem.
The abstract generic base class contains all that stuff that applies to all usable types of T, but it isn't instantiable. The concrete implementations ensure that only those T are usable that make sense to be used.

public abstract class CGeometryCalibration2D<T> : CGeometryCalibration<T>
{
  ctor()
  public override void Clear ()
  public bool Add (double i_dPosition, CGeometryCalibration1D<T> i_oGeoCalib1D)
  public bool Remove (double i_dPosition)
  public override void CopyTo (ref CGeometryCalibration<T> io_oGeoCalib)
  public override T Interpolate (CLocation i_locPosition, bool i_bExtrapolate = false)
  public T Interpolate (double i_dPosition, double i_dPositionGC1D, bool i_bExtrapolate = false)
  public override CResulT Load (CXmlDocReader i_xmldocreader)
  public override CResulT Save (CXmlDocWriter i_xmldocwriter)
}

public class CGeometryCalibration2D_Double : CGeometryCalibration2D<double>
{
  ctor()
}

public class CGeometryCalibration2D_CLocation : CGeometryCalibration2D<CLocation>
{
  ctor()
  public double InterpolateReverse (CLocation i_locValue, out CLocation o_ValueInterpolated, bool i_bExtrapolate = false)
}