Make Assembly.Load() throw same exception as CoreCLR (#4376)

When an assembly matching the simple name is found
but the version is wrong, Assembly.Load throws
a FileLoadException with a ref-def mismatch string.

This change makes Project N match that behavior as
well as separating the notion of a preferred exception
(the most informative exception to throw if no
matching candidate has found later in the search)
and the actual exception (used to signal that load
as a whole has failed.)

1 files changed, 17 insertions, 7 deletions

diff --git a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Dispensers.cs b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Dispensers.cs
index 19201f5a1..fd9603606 100644
--- a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Dispensers.cs
+++ b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Dispensers.cs
@@ -67,20 +67,30 @@ namespace System.Reflection.Runtime.Assemblies
         /// </summary>
         internal static RuntimeAssembly GetRuntimeAssemblyIfExists(RuntimeAssemblyName assemblyRefName)
         {
-            return s_assemblyRefNameToAssemblyDispenser.GetOrAdd(assemblyRefName);
+            object runtimeAssemblyOrException = s_assemblyRefNameToAssemblyDispenser.GetOrAdd(assemblyRefName);
+            if (runtimeAssemblyOrException is RuntimeAssembly runtimeAssembly)
+                return runtimeAssembly;
+            return null;
         }
 
         internal static Exception TryGetRuntimeAssembly(RuntimeAssemblyName assemblyRefName, out RuntimeAssembly result)
         {
-            result = GetRuntimeAssemblyIfExists(assemblyRefName);
-            if (result != null)
+            object runtimeAssemblyOrException = s_assemblyRefNameToAssemblyDispenser.GetOrAdd(assemblyRefName);
+            if (runtimeAssemblyOrException is RuntimeAssembly runtimeAssembly)
+            {
+                result = runtimeAssembly;
                 return null;
+            }
             else
-                return new FileNotFoundException(SR.Format(SR.FileNotFound_AssemblyNotFound, assemblyRefName.FullName));
+            {
+                result = null;
+                return (Exception)runtimeAssemblyOrException;
+            }
         }
 
-        private static readonly Dispenser<RuntimeAssemblyName, RuntimeAssembly> s_assemblyRefNameToAssemblyDispenser =
-            DispenserFactory.CreateDispenser<RuntimeAssemblyName, RuntimeAssembly>(
+        // The "object" here is either a RuntimeAssembly or an Exception.
+        private static readonly Dispenser<RuntimeAssemblyName, object> s_assemblyRefNameToAssemblyDispenser =
+            DispenserFactory.CreateDispenser<RuntimeAssemblyName, object>(
                 DispenserScenario.AssemblyRefName_Assembly,
                 delegate (RuntimeAssemblyName assemblyRefName)
                 {
@@ -88,7 +98,7 @@ namespace System.Reflection.Runtime.Assemblies
                     AssemblyBindResult bindResult;
                     Exception exception;
                     if (!binder.Bind(assemblyRefName, out bindResult, out exception))
-                        return null;
+                        return exception;
 
                     return GetRuntimeAssembly(bindResult);
                 }