Pretty print type names in converter exceptions

src/Vertical/CommandLine/Conversion/ConversionException.cs

@@ -48,7 +48,9 @@ internal ConversionException(string context, Type targetType, string argumentVal
         // Formats the exception message.
         private static string FormatMessage(string context, Type targetType, string argumentValue)
         {
-            return $"{context}: could not convert {Formatting.Quote(argumentValue)} to target type {targetType.Name}.";
+            var friendlyTargetType = TypeHelpers.GetFriendlyDisplayName(targetType);
+
+            return $"{context}: could not convert {Formatting.Quote(argumentValue)} to target type {friendlyTargetType}.";
         }
     }
 }

src/Vertical/CommandLine/Infrastructure/TypeHelpers.cs

@@ -101,5 +101,28 @@ internal static string GetGenericTypeName(string typeName)
             var markerIndex = typeName.IndexOf('`');
             return markerIndex > -1 ? typeName.Substring(0, markerIndex) : typeName;
         }
+
+        internal static string GetFriendlyDisplayName(Type type)
+        {
+            try
+            {
+                if (!type.IsGenericType)
+                    return type.Name;
+
+                var genericTypeName = type.GetGenericTypeDefinition().Name;
+                var tickIndex = genericTypeName.IndexOf('`');
+                var trimmedTypeName = genericTypeName.Substring(0, tickIndex);
+                var typeParams = type
+                    .GetGenericArguments()
+                    .Select(GetFriendlyDisplayName);
+                var typeParamString = string.Join(",", typeParams);
+
+                return $"{trimmedTypeName}<{typeParamString}>";
+            }
+            catch
+            {
+                return type.Name;
+            }
+        }
     }
 }

test/Infrastructure/TypeHelpersTests.cs

@@ -5,6 +5,7 @@
 // or refer to https://opensource.org/licenses/MIT
 
 using System;
+using System.Collections.Generic;
 using Shouldly;
 using Vertical.CommandLine.Infrastructure;
 using Xunit;
@@ -20,5 +21,15 @@ public void GetKnownMethodInfoThrowsForInvalidMethod()
                 Array.Empty<Type>(),
                 typeof(void)));
         }
+
+        [Theory]
+        [InlineData(typeof(string), "String")]
+        [InlineData(typeof(int), "Int32")]
+        [InlineData(typeof(int?), "Nullable<Int32>")]
+        [InlineData(typeof(Dictionary<int?, string>), "Dictionary<Nullable<Int32>,String>")]
+        public void GetFriendlyNameReturnsExpected(Type type, string expected)
+        {
+            TypeHelpers.GetFriendlyDisplayName(type).ShouldBe(expected);
+        }
     }
 }