Unity中的拆/装箱操作引发的GC问题解决方案

Unity C#开发中，常常用到Enum或Struct等类型作为Dictionary的key，或List的元素，即：

List<EnumType>，Dictionary<EnumType, ValueType>，List<StructType>，Dictionary<StructType, ValueType>。

在List的Contains()、Dictionary的ContainsKey()或[]操作时，会调用Equals()和GetHasCode()，从而对Enum或Struct类型进行装箱操作，产生GC问题。

解决方案：

1、Struct，override GetHasCode()并实现IEquatable<T>即可。例如：

/// <summary>
        /// sizeof(KeyStruct) = 12B
        /// 测试Dictionary的Contains()和[]
        /// 1.不override GetHashCode()和实现IEquatable<T>
        ///     Contains():84B
        ///     []:84B
        /// 2.只实现实现IEquatable<T>
        ///     Contains():28B
        ///     []:28B
        ///     []:84B
        /// 3.只override GetHashCode()
        ///     Contains():56B
        ///     []:56B
        /// 4.override GetHashCode()和实现IEquatable<T>
        ///     Contains():0
        ///     []:0
        /// </summary>
        struct KeyStruct : IEquatable<KeyStruct>
        {
            public int a;
            public int b;
            public int c;

            public KeyStruct(int a, int b, int c)
            {
                this.a = a;
                this.b = b;
                this.c = c;
            }

            public override int GetHashCode()
            {
                return this.a * 100 + this.b * 10 + this.c;
            }

            public bool Equals(KeyStruct obj)
            {
                return this.a == obj.a && this.b == obj.b && this.c == obj.c;
            }
        }

2、Enum类型由于不能override和implement接口，需要使用其他方法。

当作为Dictionary key使用时，可以为Enum定义一个专用比较器，如下：

        enum KeyEnum
        {
            K1,
            K2,
            K3,
            K4,
        }

        struct KeyEnumComparer : IEqualityComparer<KeyEnum>
        {
            public bool Equals(KeyEnum x, KeyEnum y)
            {
                return x == y;
            }

            public int GetHashCode(KeyEnum obj)
            {
                // you need to do some thinking here,
                return (int)obj;
            }
        }

        Dictionary<KeyEnum, KeyEnum> enumDict2 = new Dictionary<KeyEnum, KeyEnum>(new KeyEnumComparer())
        {
            {KeyEnum.K1,KeyEnum.K1},
            {KeyEnum.K2,KeyEnum.K1},
            {KeyEnum.K3,KeyEnum.K1},
            {KeyEnum.K4,KeyEnum.K1}
        };

List中使用Enum，由于无法在构造函数中传入自定义比较器，推荐使用List.Exists(e => e == KeyEnum.K1)代替List.Contains(KeyEnum.K1)，以避免Equals()调用。