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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Reflection;
using System.Reflection.Runtime.TypeInfos;
using Internal.Reflection.Core;
namespace System.Reflection.Runtime.General
{
internal static class Assignability
{
[UnconditionalSuppressMessage("ReflectionAnalysis", "IL2055:UnrecognizedReflectionPattern",
Justification = "Just instantiating over formals for desktop compat reasons")]
[UnconditionalSuppressMessage("AotAnalysis", "IL3050:AotUnfriendlyApi",
Justification = "Just instantiating over formals for desktop compat reasons")]
public static bool IsAssignableFrom(Type toTypeInfo, Type fromTypeInfo)
{
if (toTypeInfo == null)
throw new NullReferenceException();
if (fromTypeInfo == null)
return false; // It would be more appropriate to throw ArgumentNullException here, but returning "false" is the desktop-compat behavior.
if (fromTypeInfo.Equals(toTypeInfo))
return true;
if (toTypeInfo.IsGenericTypeDefinition)
{
// Asking whether something can cast to a generic type definition is arguably meaningless. The desktop CLR Reflection layer converts all
// generic type definitions to generic type instantiations closed over the formal generic type parameters. The .NET Native framework
// keeps the two separate. Fortunately, under either interpretation, returning "false" unless the two types are identical is still a
// defensible behavior. To avoid having the rest of the code deal with the differing interpretations, we'll short-circuit this now.
return false;
}
if (fromTypeInfo.IsGenericTypeDefinition)
{
// The desktop CLR Reflection layer converts all generic type definitions to generic type instantiations closed over the formal
// generic type parameters. The .NET Native framework keeps the two separate. For the purpose of IsAssignableFrom(),
// it makes sense to unify the two for the sake of backward compat. We'll just make the transform here so that the rest of code
// doesn't need to know about this quirk.
fromTypeInfo = fromTypeInfo.GetGenericTypeDefinition().MakeGenericType(fromTypeInfo.GetGenericTypeParameters());
}
if (fromTypeInfo.CanCastTo(toTypeInfo))
return true;
// Desktop compat: IsAssignableFrom() considers T as assignable to Nullable<T> (but does not check if T is a generic parameter.)
if (!fromTypeInfo.IsGenericParameter)
{
Type? nullableUnderlyingType = Nullable.GetUnderlyingType(toTypeInfo);
if (nullableUnderlyingType != null && nullableUnderlyingType.Equals(fromTypeInfo))
return true;
}
return false;
}
[UnconditionalSuppressMessage("ReflectionAnalysis", "IL2070:UnrecognizedReflectionPattern",
Justification = "Looking at interface list is safe because we wouldn't remove reflection-visible interface from a reflection-visible type")]
private static bool CanCastTo(this Type fromTypeInfo, Type toTypeInfo)
{
if (fromTypeInfo.Equals(toTypeInfo))
return true;
if (fromTypeInfo.IsArray)
{
if (toTypeInfo.IsInterface)
return fromTypeInfo.CanCastArrayToInterface(toTypeInfo);
if (fromTypeInfo.IsSubclassOf(toTypeInfo))
return true; // T[] is castable to Array or Object.
if (!toTypeInfo.IsArray)
return false;
int rank = fromTypeInfo.GetArrayRank();
if (rank != toTypeInfo.GetArrayRank())
return false;
bool fromTypeIsSzArray = fromTypeInfo.IsSZArray;
bool toTypeIsSzArray = toTypeInfo.IsSZArray;
if (fromTypeIsSzArray != toTypeIsSzArray)
{
// T[] is assignable to T[*] but not vice-versa.
if (!(rank == 1 && !toTypeIsSzArray))
{
return false; // T[*] is not castable to T[]
}
}
Type toElementTypeInfo = toTypeInfo.GetElementType()!;
Type fromElementTypeInfo = fromTypeInfo.GetElementType()!;
return fromElementTypeInfo.IsElementTypeCompatibleWith(toElementTypeInfo);
}
if (fromTypeInfo.IsByRef)
{
if (!toTypeInfo.IsByRef)
return false;
Type toElementTypeInfo = toTypeInfo.GetElementType()!;
Type fromElementTypeInfo = fromTypeInfo.GetElementType()!;
return fromElementTypeInfo.IsElementTypeCompatibleWith(toElementTypeInfo);
}
if (fromTypeInfo.IsPointer)
{
if (!toTypeInfo.IsPointer)
return false;
Type toElementTypeInfo = toTypeInfo.GetElementType()!;
Type fromElementTypeInfo = fromTypeInfo.GetElementType()!;
return fromElementTypeInfo.IsElementTypeCompatibleWith(toElementTypeInfo);
}
if (fromTypeInfo.IsGenericParameter)
{
//
// A generic parameter can be cast to any of its constraints, or object, if none are specified, or ValueType if the "struct" constraint is
// specified.
//
// This has to be coded as its own case as TypeInfo.BaseType on a generic parameter doesn't always return what you'd expect.
//
if (toTypeInfo == typeof(object))
return true;
if (toTypeInfo == typeof(ValueType))
{
GenericParameterAttributes attributes = fromTypeInfo.GenericParameterAttributes;
if ((attributes & GenericParameterAttributes.NotNullableValueTypeConstraint) != 0)
return true;
}
foreach (Type constraintType in fromTypeInfo.GetGenericParameterConstraints())
{
if (constraintType.CanCastTo(toTypeInfo))
return true;
}
return false;
}
if (toTypeInfo.IsArray || toTypeInfo.IsByRef || toTypeInfo.IsPointer || toTypeInfo.IsGenericParameter)
return false;
if (fromTypeInfo.MatchesWithVariance(toTypeInfo))
return true;
if (toTypeInfo.IsInterface)
{
foreach (Type ifc in fromTypeInfo.GetInterfaces())
{
if (ifc.MatchesWithVariance(toTypeInfo))
return true;
}
return false;
}
else
{
// Interfaces are always castable to System.Object. The code below will not catch this as interfaces report their BaseType as null.
if (toTypeInfo == typeof(object) && fromTypeInfo.IsInterface)
return true;
Type walk = fromTypeInfo;
for (; ; )
{
Type? baseType = walk.BaseType;
if (baseType == null)
return false;
walk = baseType;
if (walk.MatchesWithVariance(toTypeInfo))
return true;
}
}
}
//
// Check a base type or implemented interface type for equivalence (taking into account variance for generic instantiations.)
// Does not check ancestors recursively.
//
private static bool MatchesWithVariance(this Type fromTypeInfo, Type toTypeInfo)
{
Debug.Assert(!(fromTypeInfo.IsArray || fromTypeInfo.IsByRef || fromTypeInfo.IsPointer || fromTypeInfo.IsGenericParameter));
Debug.Assert(!(toTypeInfo.IsArray || toTypeInfo.IsByRef || toTypeInfo.IsPointer || toTypeInfo.IsGenericParameter));
if (fromTypeInfo.Equals(toTypeInfo))
return true;
if (!(fromTypeInfo.IsConstructedGenericType && toTypeInfo.IsConstructedGenericType))
return false;
Type genericTypeDefinition = fromTypeInfo.GetGenericTypeDefinition();
if (!genericTypeDefinition.Equals(toTypeInfo.GetGenericTypeDefinition()))
return false;
Type[] fromTypeArguments = fromTypeInfo.GenericTypeArguments;
Type[] toTypeArguments = toTypeInfo.GenericTypeArguments;
Type[] genericTypeParameters = genericTypeDefinition.GetGenericTypeParameters();
for (int i = 0; i < genericTypeParameters.Length; i++)
{
Type fromTypeArgumentInfo = fromTypeArguments[i];
Type toTypeArgumentInfo = toTypeArguments[i];
GenericParameterAttributes attributes = genericTypeParameters[i].GenericParameterAttributes;
switch (attributes & GenericParameterAttributes.VarianceMask)
{
case GenericParameterAttributes.Covariant:
if (!(fromTypeArgumentInfo.IsGcReferenceTypeAndCastableTo(toTypeArgumentInfo)))
return false;
break;
case GenericParameterAttributes.Contravariant:
if (!(toTypeArgumentInfo.IsGcReferenceTypeAndCastableTo(fromTypeArgumentInfo)))
return false;
break;
case GenericParameterAttributes.None:
if (!(fromTypeArgumentInfo.Equals(toTypeArgumentInfo)))
return false;
break;
default:
throw new BadImageFormatException(); // Unexpected variance value in metadata.
}
}
return true;
}
//
// A[] can cast to B[] if one of the following are true:
//
// A can cast to B under variance rules.
//
// A and B are both integers or enums and have the same reduced type (i.e. represent the same-sized integer, ignoring signed/unsigned differences.)
// "char" is not interchangeable with short/ushort. "bool" is not interchangeable with byte/sbyte.
//
// For desktop compat, A& and A* follow the same rules.
//
private static bool IsElementTypeCompatibleWith(this Type fromTypeInfo, Type toTypeInfo)
{
if (fromTypeInfo.IsGcReferenceTypeAndCastableTo(toTypeInfo))
return true;
Type reducedFromType = fromTypeInfo.ReducedType();
Type reducedToType = toTypeInfo.ReducedType();
if (reducedFromType.Equals(reducedToType))
return true;
return false;
}
private static Type ReducedType(this Type t)
{
if (t.IsEnum)
t = Enum.GetUnderlyingType(t);
if (t == typeof(byte))
return typeof(sbyte);
if (t == typeof(ushort))
return typeof(short);
if (t == typeof(uint))
return typeof(int);
if (t == typeof(ulong))
return typeof(long);
if (t == typeof(UIntPtr) || t == typeof(IntPtr))
{
#if TARGET_64BIT
return typeof(long);
#else
return typeof(int);
#endif
}
return t;
}
//
// Contra/CoVariance.
//
// IEnumerable<D> can cast to IEnumerable<B> if D can cast to B and if there's no possibility that D is a value type.
//
private static bool IsGcReferenceTypeAndCastableTo(this Type fromTypeInfo, Type toTypeInfo)
{
if (fromTypeInfo.Equals(toTypeInfo))
return true;
if (fromTypeInfo.ProvablyAGcReferenceType())
return fromTypeInfo.CanCastTo(toTypeInfo);
return false;
}
//
// A true result indicates that a type can never be a value type. This is important when testing variance-compatibility.
//
private static bool ProvablyAGcReferenceType(this Type t)
{
if (t.IsGenericParameter)
{
GenericParameterAttributes attributes = t.GenericParameterAttributes;
if ((attributes & GenericParameterAttributes.ReferenceTypeConstraint) != 0)
return true; // generic parameter with a "class" constraint.
}
return t.ProvablyAGcReferenceTypeHelper();
}
private static bool ProvablyAGcReferenceTypeHelper(this Type t)
{
if (t.IsArray)
return true;
if (t.IsByRef || t.IsPointer)
return false;
if (t.IsGenericParameter)
{
// We intentionally do not check for a "class" constraint on generic parameter ancestors.
// That's because this property does not propagate up the constraining hierarchy.
// (e.g. "class A<S, T> where S : T, where T : class" does not guarantee that S is a class.)
foreach (Type constraintType in t.GetGenericParameterConstraints())
{
if (constraintType.ProvablyAGcReferenceTypeHelper())
return true;
}
return false;
}
return t.IsClass && t != typeof(object) && t != typeof(ValueType) && t != typeof(Enum);
}
//
// T[] casts to IList<T>. This could be handled by the normal ancestor-walking code
// but for one complication: T[] also casts to IList<U> if T[] casts to U[].
//
[UnconditionalSuppressMessage("ReflectionAnalysis", "IL2070:UnrecognizedReflectionPattern",
Justification = "Looking at interface list is safe because we wouldn't remove reflection-visible interface from a reflection-visible type")]
private static bool CanCastArrayToInterface(this Type fromTypeInfo, Type toTypeInfo)
{
Debug.Assert(fromTypeInfo.IsArray);
Debug.Assert(toTypeInfo.IsInterface);
if (toTypeInfo.IsConstructedGenericType)
{
Type[] toTypeGenericTypeArguments = toTypeInfo.GenericTypeArguments;
if (toTypeGenericTypeArguments.Length != 1)
return false;
Type toElementTypeInfo = toTypeGenericTypeArguments[0];
Type toTypeGenericTypeDefinition = toTypeInfo.GetGenericTypeDefinition();
Type fromElementTypeInfo = fromTypeInfo.GetElementType()!;
foreach (Type ifc in fromTypeInfo.GetInterfaces())
{
if (ifc.IsConstructedGenericType)
{
Type ifcGenericTypeDefinition = ifc.GetGenericTypeDefinition();
if (ifcGenericTypeDefinition.Equals(toTypeGenericTypeDefinition))
{
if (fromElementTypeInfo.IsElementTypeCompatibleWith(toElementTypeInfo))
return true;
}
}
}
return false;
}
else
{
foreach (Type ifc in fromTypeInfo.GetInterfaces())
{
if (ifc.Equals(toTypeInfo))
return true;
}
return false;
}
}
}
}
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