[csharp] OneOf should handle Inheritance based type

[Ghufz]

In the current C# generator, properties that have a oneOf relationship only handle the relationship for concrete types. However, if a schema in the oneOf clause is an abstract type, it should accept types derived from the abstract class. Unfortunately, this behavior is not possible in the current C# generator. I’ve implemented a fix to handle inheritance-based type checks using the is operator.

The current generator generate a code as below for oneOf schema

public override Object ActualInstance
        {
            get
            {
                return _actualInstance;
            }
            set
            {
                if (value.GetType() == typeof(FabricAbstractInterfaceRole))
                {
                    this._actualInstance = value;
                }
                else if (value.GetType() == typeof(MoMoRef) )
                {
                    this._actualInstance = value;
                }
                else
                {
                    throw new ArgumentException("Invalid instance found. Must be the following types: FabricAbstractInterfaceRole, MoMoRef");
                }
            }
        }

Expected behaviour to accepts any type which are derived from abstract class

public override Object ActualInstance
        {
            get
            {
                return _actualInstance;
            }
            set
            {
                if (value.GetType() == typeof(FabricAbstractInterfaceRole) || value is FabricAbstractInterfaceRole)
                {
                    this._actualInstance = value;
                }
                else if (value.GetType() == typeof(MoMoRef) || value is MoMoRef )
                {
                    this._actualInstance = value;
                }
                else
                {
                    throw new ArgumentException("Invalid instance found. Must be the following types: FabricAbstractInterfaceRole, MoMoRef");
                }
            }
        }

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  @wing328

[wing328]

thanks for the PR

cc
@mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50026252