Update a few more tests

src/FlatSharp.Compiler/SchemaModel/ValueUnionSchemaModel.cs

@@ -190,7 +190,7 @@ protected override void OnWriteCode(CodeWriter writer, CompileContext context)
                         writer.AppendLine($"if (this.Discriminator != {item.value.Value})");
                         using (writer.WithBlock())
                         {
-                            writer.AppendLine($"{typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}(\"Union Discriminator != {item.value.Value}\");");
+                            writer.AppendLine($"{typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_UnionIsNotOfType)}();");
                         }
 
                         writer.AppendLine($"return this.UncheckedGetItem{item.value.Value}();");
@@ -257,7 +257,12 @@ private void WriteAcceptMethod(
                     writer.AppendLine($"case {index}: return visitor.Visit(this.UncheckedGetItem{item.value.Value}());");
                 }
 
-                writer.AppendLine($"default: return {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}<TReturn>(\"Unexpected discriminator\");");
+                writer.AppendLine($"default:");
+                using (writer.IncreaseIndent())
+                {
+                    writer.AppendLine($"{typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_InvalidUnionDiscriminator)}<{this.Name}>(disc);");
+                    writer.AppendLine("return default(TReturn);");
+                }
             }
         }
     }

src/FlatSharp.Runtime/FSThrow.cs

@@ -24,12 +24,25 @@ public static class FSThrow
     #region InvalidOperation
 
     [DoesNotReturn]
-    public static void InvalidOperation(string message) => throw new InvalidOperationException(message);
+    public static void InvalidOperation(string message)
+        => throw new InvalidOperationException(message);
 
     [DoesNotReturn]
     public static void InvalidOperation_SizeNotMultipleOfAlignment(Type elementType, int size, int alignment)
         => throw new InvalidOperationException($"Type '{elementType.FullName}' does not support Unsafe Span operations because the size ({size}) is not a multiple of the alignment ({alignment}).");
 
+    [DoesNotReturn]
+    public static void InvalidOperation_RequiredPropertyNotSet(string propertyName)
+        => throw new InvalidOperationException($"Table property '{propertyName}' is marked as required, but was not set.");
+
+    [DoesNotReturn]
+    public static T InvalidOperation_InvalidUnionDiscriminator<T>(byte discriminator)
+        => throw new InvalidOperationException($"Unexpected union discriminator value '{discriminator}' for Union {typeof(T).FullName}");
+
+    [DoesNotReturn]
+    public static void InvalidOperation_UnionIsNotOfType()
+        => throw new InvalidOperationException("The union is not of the requested type.");
+
     [DoesNotReturn]
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static T InvalidOperation<T>(string message)
@@ -43,11 +56,11 @@ public static T InvalidOperation<T>(string message)
     #region InvalidData
 
     [DoesNotReturn]
-    public static void InvalidData_UOffsetTooSmall(uint uoffset) 
+    public static void InvalidData_UOffsetTooSmall(uint uoffset)
         => throw new InvalidDataException($"FlatBuffer was in an invalid format: Decoded uoffset_t had value less than {sizeof(uint)}. Value = {uoffset}");
 
     [DoesNotReturn]
-    public static void InvalidData_VTableTooShort() 
+    public static void InvalidData_VTableTooShort()
         => throw new InvalidDataException("FlatBuffer was in an invalid format: VTable was not long enough to be valid.");
 
     [DoesNotReturn]
@@ -58,6 +71,10 @@ public static void InvalidData_InvalidNull()
     public static void InvalidData_DepthLimit()
         => throw new InvalidDataException("FlatSharp passed the configured depth limit when deserializing. This can be configured with 'IGeneratedSerializer.WithSettings'.");
 
+    [DoesNotReturn]
+    public static void InvalidData_RequiredPropertyNotSet(string propertyName)
+        => throw new InvalidDataException($"Table property '{propertyName}' is marked as required, but was missing from the buffer.");
+
     [DoesNotReturn]
     public static void InvalidData(string message) => throw new InvalidDataException(message);
 

src/FlatSharp/TypeModel/TableMemberModel.cs

@@ -254,8 +254,7 @@ private string GetNotPresentStatement()
     {
         if (this.IsRequired)
         {
-            string message = $"Table property '{this.FriendlyName}' is marked as required, but was missing from the buffer.";
-            return $"{typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidData)}(\"{message}\");";
+            return $"{typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidData_RequiredPropertyNotSet)}(\"{this.FriendlyName}\");";
         }
         else
         {

src/FlatSharp/TypeModel/TableTypeModel.cs

@@ -540,7 +540,7 @@ private string GetPrepareSerializeBlock(
             elseBlock =
             $@"else
             {{
-                {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}(""Table property '{memberModel.FriendlyName}' is marked as required, but was not set."");
+                {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_RequiredPropertyNotSet)}(""{memberModel.FriendlyName}"");
             }}
             ";
         }

src/FlatSharp/TypeModel/UnionTypeModel.cs

@@ -107,11 +107,13 @@ public override CodeGeneratedMethod CreateGetMaxSizeMethodBody(GetMaxSizeCodeGen
 
         string body =
 $@"
-            switch ({context.ValueVariableName}.{discriminatorPropertyName})
+            byte discriminator = {context.ValueVariableName}.{discriminatorPropertyName};
+            switch (discriminator)
             {{
                 {string.Join("\r\n", switchCases)}
                 default:
-                    return {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}<int>(""Exception determining type of union. Unexpected Union discriminator."");
+                    {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_InvalidUnionDiscriminator)}<{this.GGCTN()}>(discriminator);
+                    return 0;
             }}
 ";
         return new CodeGeneratedMethod(body);
@@ -157,7 +159,8 @@ public override CodeGeneratedMethod CreateParseMethodBody(ParserCodeGenContext c
             {{
                 {string.Join("\r\n", switchCases)}
                 default:
-                    return {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}<{this.GGCTN()}>(""Exception parsing union '{this.GetCompilableTypeName()}'. Unexpected union discriminator."");
+                    {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_InvalidUnionDiscriminator)}<{this.GGCTN()}>(discriminator);
+                    return default({this.GGCTN()});
             }}
         ";
 
@@ -305,7 +308,7 @@ public override CodeGeneratedMethod CreateSerializeMethodBody(SerializationCodeG
             {{
                 {string.Join("\r\n", switchCases)}
                 default: 
-                    {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}(""Unexpected union discriminator. Unions must be initialized."");
+                    {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_InvalidUnionDiscriminator)}<{this.GGCTN()}>(discriminatorValue);
                     break;
             }}";
 
@@ -320,15 +323,22 @@ public override CodeGeneratedMethod CreateCloneMethodBody(CloneCodeGenContext co
         {
             int discriminator = i + 1;
             string cloneMethod = context.MethodNameMap[this.memberTypeModels[i].ClrType];
-            switchCases.Add($"{discriminator} => new {this.GetGlobalCompilableTypeName()}({cloneMethod}({context.ItemVariableName}.Item{discriminator})),");
+            switchCases.Add($@"
+                case {discriminator}:
+                    return new {this.GetGlobalCompilableTypeName()}({cloneMethod}({context.ItemVariableName}.Item{discriminator}));
+                ");
         }
 
-        switchCases.Add($"_ => {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation)}<{this.GGCTN()}>(\"Unexpected union discriminator\")");
-
         string body = $@"
-            return {context.ItemVariableName}.{nameof(IFlatBufferUnion.Discriminator)} switch {{
+            byte discriminator = {context.ItemVariableName}.{nameof(IFlatBufferUnion.Discriminator)};
+            switch (discriminator)
+            {{
                 {string.Join("\r\n", switchCases)}
-            }};";
+                default: 
+                    {typeof(FSThrow).GGCTN()}.{nameof(FSThrow.InvalidOperation_InvalidUnionDiscriminator)}<{this.GGCTN()}>(discriminator);
+                    return default({this.GGCTN()});
+            }}
+            ";
 
         return new CodeGeneratedMethod(body);
     }

src/Tests/Stryker/Tests/UnionFieldTests.cs

@@ -20,17 +20,17 @@ public void InvalidGetters()
         FunUnion a = new FunUnion(new RefStruct());
 
         var ex = Assert.Throws<InvalidOperationException>(() => a.ValueStruct);
-        Assert.Equal("Union Discriminator != 2", ex.Message);
+        Assert.Equal("The union is not of the requested type.", ex.Message);
 
         ex = Assert.Throws<InvalidOperationException>(() => a.str);
-        Assert.Equal("Union Discriminator != 3", ex.Message);
+        Assert.Equal("The union is not of the requested type.", ex.Message);
 
         ex = Assert.Throws<InvalidOperationException>(() => a.Key);
-        Assert.Equal("Union Discriminator != 4", ex.Message);
+        Assert.Equal("The union is not of the requested type.", ex.Message);
 
         a = new FunUnion(new ValueStruct());
         ex = Assert.Throws<InvalidOperationException>(() => a.RefStruct);
-        Assert.Equal("Union Discriminator != 1", ex.Message);
+        Assert.Equal("The union is not of the requested type.", ex.Message);
     }
 
     [Fact]
@@ -75,19 +75,21 @@ public void StringMember(FlatBufferDeserializationOption option)
     [Fact]
     public void BrokenUnion()
     {
+        string message = $"Unexpected union discriminator value '0' for Union {typeof(FunUnion).FullName}";
+
         Root root = new() { Fields = new() { Union = new FunUnion() } };
 
         var ex = Assert.Throws<InvalidOperationException>(() => Root.Serializer.GetMaxSize(root));
-        Assert.Equal("Exception determining type of union. Unexpected Union discriminator.", ex.Message);
+        Assert.Equal(message, ex.Message);
 
         ex = Assert.Throws<InvalidOperationException>(() => Root.Serializer.Write(new byte[1024], root));
-        Assert.Equal("Unexpected union discriminator. Unions must be initialized.", ex.Message);
+        Assert.Equal(message, ex.Message);
 
         ex = Assert.Throws<InvalidOperationException>(() => new FunUnion().Accept<Visitor, bool>(new Visitor()));
-        Assert.Equal("Unexpected discriminator", ex.Message);
+        Assert.Equal(message, ex.Message);
 
         ex = Assert.Throws<InvalidOperationException>(() => new Root(root));
-        Assert.Equal("Unexpected union discriminator", ex.Message);
+        Assert.Equal(message, ex.Message);
     }
 
     private struct Visitor : FunUnion.Visitor<bool>
@@ -238,7 +240,9 @@ public void InvalidUnion_InvalidDiscriminator(FlatBufferDeserializationOption op
             FunUnion union = root.Fields.Union.Value;
         });
 
-        Assert.Equal("Exception parsing union 'FlatSharpStrykerTests.FunUnion'. Unexpected union discriminator.", ex.Message);
+        Assert.Equal(
+            $"Unexpected union discriminator value '10' for Union {typeof(FunUnion).FullName}",
+            ex.Message);
     }
 
     [Theory]

src/Tests/Stryker/Tests/UnionVectorTests.cs

@@ -158,7 +158,9 @@ public void UnionVector_Invalid_InvalidDiscriminator(FlatBufferDeserializationOp
             var x = item.Vectors.Union[0];
         });
 
-        Assert.Equal("Exception parsing union 'FlatSharpStrykerTests.FunUnion'. Unexpected union discriminator.", ex.Message);
+        Assert.Equal(
+            $"Unexpected union discriminator value '7' for Union {typeof(FunUnion).FullName}",
+            ex.Message);
     }
 
     [Theory]