Overriding Equals method in Structs

问题:

I've looked for overriding guidelines for structs, but all I can find is for classes.

At first I thought I wouldn't have to check to see if the passed object was null, as structs are value types and can't be null. But now that I come to think of it, as equals signature is

public bool Equals(object obj)

it seems there is nothing preventing the user of my struct to be trying to compare it with an arbitrary reference type.

My second point concerns the casting I (think I) have to make before I compare my private fields in my struct. How am I supposed to cast the object to my struct's type? C#'s as keyword seems only suitable for reference types.

回答1:

struct MyStruct 
{
   public override bool Equals(object obj) 
   {
       if (!(obj is MyStruct))
          return false;

       MyStruct mys = (MyStruct) obj;
       // compare elements here

   }

}

回答2:

Thanks to pattern matching in C# 7.0 there is an easier way to accomplish the accepted answer:

struct MyStruct 
{
    public override bool Equals(object obj) 
    {
        if (!(obj is MyStruct mys)) // type pattern here
            return false;

        return this.field1 == mys.field1 && this.field2 == mys.field2 // mys is already known here without explicit casting
    }
}

You could also make it even shorter as an expression-bodied function:

struct MyStruct 
{
    public override bool Equals(object obj) => 
        obj is MyStruct mys
            && mys.field1 == this.field1
            && mys.field2 == this.field2;
}

回答3:

I suppose, if one's using .NET 4.5, one can use the default implementation as noted in the documentation:

When you define your own type, that type inherits the functionality defined by the Equals method of its base type.

ValueType.Equals: Value equality; either direct byte-by-byte comparison or field-by-field comparison using reflection.

回答4:

In case anyone's wondering about the performance hit of boxing the struct in a Nullable object (to avoid the double type check from is and the cast), there is a non-negligible overhead.

tl;dr: Use is & cast in this scenario.

struct Foo : IEquatable<Foo>
{
    public int a, b;

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

    public override bool Equals(object obj)
    {
#if BOXING
        var obj_ = obj as Foo?;
        return obj_ != null && Equals(obj_.Value);
#elif DOUBLECHECK
        return obj is Foo && Equals((Foo)obj);
#elif MAGIC
        ?
#endif
    }

    public bool Equals(Foo other)
    {
        return a == other.a && b == other.b;
    }
}

class Program
{
    static void Main(string[] args)
    {
        RunBenchmark(new Foo(42, 43), new Foo(42, 43));
        RunBenchmark(new Foo(42, 43), new Foo(43, 44));
    }

    static void RunBenchmark(object x, object y)
    {
        var sw = Stopwatch.StartNew();
        for (var i = 0; i < 100000000; i++) x.Equals(y);
        sw.Stop();
        Console.WriteLine(sw.ElapsedMilliseconds);
    }
}

Results:

BOXING
EQ  8012    7973    7981    8000
NEQ 7929    7715    7906    7888

DOUBLECHECK
EQ  3654    3650    3638    3605
NEQ 3310    3301    3319    3297

Warning: This test might be flawed in many ways, though I did verify that the benchmark code itself wasn't optimized in an odd fashion.

Looking at the IL, the double-check method compiles a little cleaner.

Boxing IL:

.method public hidebysig virtual 
    instance bool Equals (
        object obj
    ) cil managed 
{
    // Method begins at RVA 0x2060
    // Code size 37 (0x25)
    .maxstack 2
    .locals init (
        [0] valuetype [mscorlib]System.Nullable`1<valuetype StructIEqualsImpl.Foo> obj_
    )

    IL_0000: ldarg.1
    IL_0001: isinst valuetype [mscorlib]System.Nullable`1<valuetype StructIEqualsImpl.Foo>
    IL_0006: unbox.any valuetype [mscorlib]System.Nullable`1<valuetype StructIEqualsImpl.Foo>
    IL_000b: stloc.0
    IL_000c: ldloca.s obj_
    IL_000e: call instance bool valuetype [mscorlib]System.Nullable`1<valuetype StructIEqualsImpl.Foo>::get_HasValue()
    IL_0013: brfalse.s IL_0023

    IL_0015: ldarg.0
    IL_0016: ldloca.s obj_
    IL_0018: call instance !0 valuetype [mscorlib]System.Nullable`1<valuetype StructIEqualsImpl.Foo>::get_Value()
    IL_001d: call instance bool StructIEqualsImpl.Foo::Equals(valuetype StructIEqualsImpl.Foo)
    IL_0022: ret

    IL_0023: ldc.i4.0
    IL_0024: ret
} // end of method Foo::Equals

Double-check IL:

.method public hidebysig virtual 
    instance bool Equals (
        object obj
    ) cil managed 
{
    // Method begins at RVA 0x2060
    // Code size 23 (0x17)
    .maxstack 8

    IL_0000: ldarg.1
    IL_0001: isinst StructIEqualsImpl.Foo
    IL_0006: brfalse.s IL_0015

    IL_0008: ldarg.0
    IL_0009: ldarg.1
    IL_000a: unbox.any StructIEqualsImpl.Foo
    IL_000f: call instance bool StructIEqualsImpl.Foo::Equals(valuetype StructIEqualsImpl.Foo)
    IL_0014: ret

    IL_0015: ldc.i4.0
    IL_0016: ret
} // end of method Foo::Equals

Props to Roman Reiner for spotting a mistake that really wasn't making me look good.

回答5:

Use the is operator:

public bool Equals(object obj)
{
  if (obj is MyStruct)
  {
    var o = (MyStruct)obj;
    ...
  }
}

回答6:

Adding to the existing answers.

You can still have nullable values if you append a ? after the struct name (this works for every value object)

int?

Casting is done also by calling (MyStructName)variableName