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using System;
unsafe struct S
{
public void* ptr;
public object obj;
static void Main()
{
S s = default(S);
Console.WriteLine(Equals(s, s));
}
}
@controlflow
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controlflow commented Jan 15, 2014

using System;

unsafe struct S
{
  public object obj;
  public void* ptr;

  static void Main()
  {
    S s = default(S);

    var ptrField = typeof(S).GetField("ptr");

    Console.WriteLine(ptrField.FieldType.FullName); // System.Reflection.Pointer

    var value = ptrField.GetValue(s);
    var otherValue = ptrField.GetValue(s);
    Console.WriteLine(Equals(value, otherValue)); // false, two different boxed instances

    Console.WriteLine(Equals(s, s));
  }
}

When commenting one of the fields, default Equals implementation uses byte-to-byte comparison.
Otherwise, it walks over all the fields with reflection. Since pointer types are not Object-derived (to be exposed from FieldInfo.GetValue()), reflection API packs them into instances of System.Reflection.Pointer ref type. Unfortunately, this type do not overrides Equals() implementation to provide value equality semantics.

@SergeyTeplyakov
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SergeyTeplyakov commented Jan 16, 2014

Interesting, why struct with only one pointer or with pointer and value type behaves correctly?

using System;

unsafe struct S_Ok1
{
    public void* ptr;
    public void* ptr2;
}

unsafe struct S_Ok2
{
    public int n;
    public int* ptr;
}

unsafe struct S
{
    // Could be any reference type
    public object o;
    public int* ptr;
}
class Program
{
    public static void Main()
    {
        S s = default(S);
        S_Ok1 sOk1 = default(S_Ok1);
        S_Ok2 sOk2 = default(S_Ok2);

        Console.WriteLine(Equals(s, s)); // False
        Console.WriteLine(Equals(sOk1, sOk1)); // True
        Console.WriteLine(Equals(sOk2, sOk2)); // True
    }
}

I assume that you ideas is correct, but in case when struct doesn't contains reference type fast version of Equals method is used:

// Inside ValueType.cs
bool Equals(object obj)
{
    //Compare type 

    object a = this;
    if (CanCompareBits(this))
    {
        return FastEqualsCheck(a, obj);
    } 

    //Compare using reflection
}

It seems that CanCompareBits return true only when struct contains reference type but returns true if struct contains unsafe pointer, and this comment from Rotor is not accurate:

The comment of CanCompareBits says "Return true if the valuetype does not contain pointer and is tightly packed". And FastEqualsCheck uses "memcmp" to speed up the comparison.

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