c# 避免box、unbox

1、byte、int为key

2、dictionnary

使用enum、定义自定义的struct时，实现IEquatable <T>接口

 

private static EqualityComparer<T> CreateComparer()
{
    RuntimeType runtimeType = (RuntimeType)typeof(T);
    if ((Type)runtimeType == typeof(byte))
    {
        return (EqualityComparer<T>)new ByteEqualityComparer();
    }
    if (typeof(IEquatable<T>).IsAssignableFrom(runtimeType))
    {
        return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericEqualityComparer<int>), runtimeType);
    }
    if (runtimeType.IsGenericType && runtimeType.GetGenericTypeDefinition() == typeof(Nullable<>))
    {
        RuntimeType runtimeType2 = (RuntimeType)runtimeType.GetGenericArguments()[0];
        if (typeof(IEquatable<>).MakeGenericType(runtimeType2).IsAssignableFrom(runtimeType2))
        {
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(NullableEqualityComparer<int>), runtimeType2);
        }
    }
    if (runtimeType.IsEnum)
    {
        switch (Type.GetTypeCode(Enum.GetUnderlyingType(runtimeType)))
        {
        case TypeCode.Int16:
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(ShortEnumEqualityComparer<short>), runtimeType);
        case TypeCode.SByte:
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(SByteEnumEqualityComparer<sbyte>), runtimeType);
        case TypeCode.Byte:
        case TypeCode.UInt16:
        case TypeCode.Int32:
        case TypeCode.UInt32:
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(EnumEqualityComparer<int>), runtimeType);
        case TypeCode.Int64:
        case TypeCode.UInt64:
            return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(LongEnumEqualityComparer<long>), runtimeType);
        }
    }
    return new ObjectEqualityComparer<T>();
}
internal class ObjectEqualityComparer<T> : EqualityComparer<T>
{
    public override bool Equals(T x, T y)
    {
        if (x != null)
        {
            if (y != null)
            {
                return x.Equals(y);
            }
            return false;
        }
        if (y != null)
        {
            return false;
        }
        return true;
    }

    public override int GetHashCode(T obj)
    {
        return obj?.GetHashCode() ?? 0;
    }

    internal override int IndexOf(T[] array, T value, int startIndex, int count)
    {
        int num = startIndex + count;
        if (value == null)
        {
            for (int i = startIndex; i < num; i++)
            {
                if (array[i] == null)
                {
                    return i;
                }
            }
        }
        else
        {
            for (int j = startIndex; j < num; j++)
            {
                if (array[j] != null && array[j].Equals(value))
                {
                    return j;
                }
            }
        }
        return -1;
    }

    internal override int LastIndexOf(T[] array, T value, int startIndex, int count)
    {
        int num = startIndex - count + 1;
        if (value == null)
        {
            for (int num2 = startIndex; num2 >= num; num2--)
            {
                if (array[num2] == null)
                {
                    return num2;
                }
            }
        }
        else
        {
            for (int num3 = startIndex; num3 >= num; num3--)
            {
                if (array[num3] != null && array[num3].Equals(value))
                {
                    return num3;
                }
            }
        }
        return -1;
    }

    public override bool Equals(object obj)
    {
        ObjectEqualityComparer<T> objectEqualityComparer = obj as ObjectEqualityComparer<T>;
        return objectEqualityComparer != null;
    }

    public override int GetHashCode()
    {
        return base.GetType().Name.GetHashCode();
    }
}