.NET避免装箱的方法

private struct MyKey {
    private readonly int _a;
    private readonly int _b;

    public MyKey(int a, int b) {
        _a = a;
        _b = b;
    }
}

private struct MyKey {
    // ...

    public override int GetHashCode() {
        // ...
    }

    public override bool Equals(object that) {
        // ...
    }
}

public interface IEqualityComparer<in T> {
    bool Equals(T x, T y);
    int GetHashCode(T obj);
}

private static EqualityComparer<T> CreateComparer<T>() {
    Contract.Ensures(Contract.Result<EqualityComparer<T>>() != null);

    RuntimeType t = (RuntimeType)typeof(T);
    // Specialize type byte for perfORMance reasons 
    if (t == typeof(byte)) {
        return (EqualityComparer<T>)(object)(new ByteEqualityComparer());
    }

    // If T implements IEquatable<T> return a GenericEqualityComparer<T>
    if (typeof(IEquatable<T>).IsAssignableFrom(t)) {
        return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter(
                (RuntimeType)typeof(GenericEqualityComparer<int>), t);
    }

    // If T is a Nullable<U> where U implements IEquatable<U> return a NullableEqualityComparer<U>
    if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>)) {
        RuntimeType u = (RuntimeType)t.GetGenericArguments()[0];
        if (typeof(IEquatable<>).MakeGenericType(u).IsAssignableFrom(u)) {
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter(
                    (RuntimeType)typeof(NullableEqualityComparer<int>), u);
        }
    }

    // If T is an int-based Enum, return an EnumEqualityComparer<T>
    // See the METHOD__JIT_HELPERS__UNSAFE_ENUM_CAST and METHOD__JIT_HELPERS__UNSAFE_ENUM_CAST_LONG cases in getILIntrinsicImplementation 
    if (t.IsEnum && Enum.GetUnderlyingType(t) == typeof(int)) {
        return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter(
                (RuntimeType)typeof(EnumEqualityComparer<int>), t);
    }

    // Otherwise return an ObjectEqualityComparer<T> 
    return new ObjectEqualityComparer<T>();
}

struct MyKey : IEquatable<MyKey> {
    // ...

    public bool Equals(MyKey that) {
        // ...
    }
}