fix issue with genereic list resolution for existing types (#3072)

Author: m-nash

src/AutoRest.CSharp/Common/Utilities/NamedTypeSymbolExtensions.cs

@@ -3,8 +3,6 @@
 
 using System;
 using System.Linq;
-using System.Reflection;
-using System.Runtime.Serialization;
 using AutoRest.CSharp.Generation.Types;
 using Microsoft.CodeAnalysis;
 
@@ -21,9 +19,22 @@ public static CSharpType GetCSharpType(this INamedTypeSymbol symbol)
                 return GetCSharpType((INamedTypeSymbol)symbol.TypeArguments[0]).WithNullable(true);
             }
 
+            return GetFrameworkType(symbol);
+        }
+
+        private static Type GetFrameworkType(INamedTypeSymbol symbol)
+        {
             var symbolName = symbol.ToDisplayString(FullyQualifiedNameFormat);
             var assemblyName = symbol.ContainingAssembly.Name;
-            return Type.GetType(symbolName) ?? Type.GetType($"{symbolName}, {assemblyName}") ?? throw new InvalidOperationException($"Type '{symbolName}' can't be found in assembly '{assemblyName}'.");
+            if (symbol.TypeArguments.Length > 0)
+            {
+                symbolName += $"`{symbol.TypeArguments.Length}";
+            }
+
+            var type = Type.GetType(symbolName) ?? Type.GetType($"{symbolName}, {assemblyName}") ?? throw new InvalidOperationException($"Type '{symbolName}' can't be found in assembly '{assemblyName}'.");
+            return symbol.TypeArguments.Length > 0
+                ? type.MakeGenericType(symbol.TypeArguments.Cast<INamedTypeSymbol>().Select(GetFrameworkType).ToArray())
+                : type;
         }
     }
 }