[corlib] Fix Type.GetInterface() to find implemented interface when "ignoreCase: true" is used (mono#7464)

Fixes mono#6579

Author: KonH

mcs/class/corlib/Test/System/TypeTest.cs

@@ -4886,6 +4886,30 @@ public void NullFullNameForSpecificGenericTypes()
 			}
 		}
 
+		// https://github.com/mono/mono/issues/6579
+		[Test]
+		public void GetInterfaceCaseInsensitiveTest()
+		{
+			var type = typeof(Dictionary<string, object>);
+
+			Assert.NotNull (
+				type.GetInterface ("System.Collections.IDictionary", false),
+				"strict named interface must be found (ignoreCase = false)"
+			);
+			Assert.NotNull (
+				type.GetInterface ("System.Collections.IDictionary", true),
+				"strict named interface must be found (ignoreCase = true)"
+			);
+			Assert.Null (
+				type.GetInterface ("System.Collections.Idictionary", false),
+				"interface, named in mixed case, must not be found (ignoreCase = false)"
+			);
+			Assert.NotNull (
+				type.GetInterface ("System.Collections.Idictionary", true),
+				"interface, named in mixed case, must be found (ignoreCase = true)"
+			);
+		}
+
 		interface Bug59738Interface<T> {
 		}
 

mcs/class/referencesource/mscorlib/system/rttype.cs

@@ -3413,9 +3413,12 @@ public override Type GetInterface(String fullname, bool ignoreCase)
 
 #if MONO
             List<RuntimeType> list = null;
+            var nameComparison = ignoreCase ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
             foreach (RuntimeType t in GetInterfaces ()) {
-                if (t.Name != name)
-                    continue;
+
+                if (!String.Equals(t.Name, name, nameComparison)) {
+                       continue;
+                }
 
                 if (list == null)
                     list = new List<RuntimeType> (2);