The code:
var a1 = new KeyValuePair<int, int>(1, 2);
var a2 = new KeyValuePair<int, int>(1, 3);
Console.WriteLine(a1.GetHashCode() == a2.GetHashCode());
var b1 = new KeyValuePair<int, string>(1, "x");
var b2 = new KeyValuePair<int, string>(1, "y");
Console.WriteLine(b1.GetHashCode() == b2.GetHashCode());The implementations of ValueType.GetHashCode() in MS.NET and Mono are different.
The declaration of ValueType.GetHashCode() in .NET Framework 4.5.1 in referencesource.microsoft.com/#mscorlib/system/valuetype.cs:
/*=================================GetHashCode==================================
**Action: Our algorithm for returning the hashcode is a little bit complex. We look
** for the first non-static field and get it's hashcode. If the type has no
** non-static fields, we return the hashcode of the type. We can't take the
** hashcode of a static member because if that member is of the same type as
** the original type, we'll end up in an infinite loop.
**Returns: The hashcode for the type.
**Arguments: None.
**Exceptions: None.
==============================================================================*/
[System.Security.SecuritySafeCritical] // auto-generated
[ResourceExposure(ResourceScope.None)]
[MethodImplAttribute(MethodImplOptions.InternalCall)]
public extern override int GetHashCode();The implementation of ValueType.GetHashCode() in Rotor:
public override int GetHashCode() {
// Note that for correctness, we can't use any field of the value type
// since that field may be mutable in some way. If we use that field
// and the value changes, we may not be able to look up that type in a
// hash table. For correctness, we need to use something unique to
// the type of this object.
// HOWEVER, we decided that the perf of returning a constant value (such as
// the hash code for the type) would be too big of a perf hit. We're willing
// to deal with less than perfect results, and people should still be
// encouraged to override GetHashCode.
BCLDebug.Correctness(!(this is System.Collections.DictionaryEntry), "Calling GetHashCode on DictionaryEntry is dumb and probably wrong.");
BCLDebug.Perf(false, "ValueType::GetHashCode is not fast. Perhaps "+this.GetType().FullName+" should override GetHashCode()");
RuntimeType thisType = (RuntimeType)this.GetType();
FieldInfo[] thisFields = thisType.InternalGetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, false);
if (thisFields.Length>0) {
for (int i=0; i<thisFields.Length; i++) {
Object obj = ((Object)(((RuntimeFieldInfo)thisFields[i]).InternalGetValue((Object)this,false)));
if (obj != null)
return obj.GetHashCode();
}
}
// Using the method table pointer is about 4x faster than getting the
// sync block index for the Type object.
return GetMethodTablePtrAsInt(this);
}The implementation of ValueType.GetHashCode() in Mono 3.10.0:
ves_icall_System_ValueType_InternalGetHashCode (MonoObject *this, MonoArray **fields)
{
MonoClass *klass;
MonoObject **values = NULL;
MonoObject *o;
int count = 0;
gint32 result = (int)(gsize)mono_defaults.int32_class;
MonoClassField* field;
gpointer iter;
MONO_ARCH_SAVE_REGS;
klass = mono_object_class (this);
if (mono_class_num_fields (klass) == 0)
return result;
/*
* Compute the starting value of the hashcode for fields of primitive
* types, and return the remaining fields in an array to the managed side.
* This way, we can avoid costly reflection operations in managed code.
*/
iter = NULL;
while ((field = mono_class_get_fields (klass, &iter))) {
if (field->type->attrs & FIELD_ATTRIBUTE_STATIC)
continue;
if (mono_field_is_deleted (field))
continue;
/* FIXME: Add more types */
switch (field->type->type) {
case MONO_TYPE_I4:
result ^= *(gint32*)((guint8*)this + field->offset);
break;
case MONO_TYPE_STRING: {
MonoString *s;
s = *(MonoString**)((guint8*)this + field->offset);
if (s != NULL)
result ^= mono_string_hash (s);
break;
}
default:
if (!values)
values = g_newa (MonoObject*, mono_class_num_fields (klass));
o = mono_field_get_value_object (mono_object_domain (this), field, this);
values [count++] = o;
}
}
if (values) {
int i;
mono_gc_wbarrier_generic_store (fields, (MonoObject*) mono_array_new (mono_domain_get (), mono_defaults.object_class, count));
for (i = 0; i < count; ++i)
mono_array_setref (*fields, i, values [i]);
} else {
*fields = NULL;
}
return result;
}- ms (Windows, MS.NET):
False True - mono (Linux, Mono 3.10):
False False