Define expected behaviour for value-tuple parameters and returns, and implement

Dapper.Tests/Dapper.Tests.csproj

@@ -25,6 +25,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
     <PackageReference Include="xunit.runner.console" Version="2.2.0" />
     <PackageReference Include="xunit" Version="2.2.0" />
+    <PackageReference Include="System.ValueTuple" Version="4.3.0"/>
   </ItemGroup>
   
   <ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">

Dapper.Tests/TupleTests.cs

+using System;
+using Xunit;
+
+namespace Dapper.Tests
+{
+    public class TupleTests : TestBase
+    {
+        [Fact]
+        public void TupleStructParameter_Fails_HelpfulMessage()
+        {
+            try
+            {
+                // I can see this happening...
+                connection.QuerySingle<int>("select @id", (id: 42, name: "Fred"));
+                Assert.Fail();
+                ValueTuple<int, int> b = (24, 13);
+                b.Item1.IsEqualTo(24);
+            }
+            catch (NotSupportedException ex)
+            {
+                ex.Message.IsEqualTo("ValueTuple should not be used for parameters - the language-level names are not available to use as parameter names, and it adds unnecessary boxing");
+            }
+        }
+
+        [Fact]
+        public void TupleClassParameter_Works()
+        {
+            connection.QuerySingle<int>("select @Item1", Tuple.Create(42, "Fred")).IsEqualTo(42);
+        }
+
+        [Fact]
+        public void TupleReturnValue_Works_ByPosition()
+        {
+            var val = connection.QuerySingle<(int id, string name)>("select 42, 'Fred'");
+            val.id.IsEqualTo(42);
+            val.name.IsEqualTo("Fred");
+        }
+
+        [Fact]
+        public void TupleReturnValue_Works_NamesIgnored()
+        {
+            var val = connection.QuerySingle<(int id, string name)>("select 42 as [Item2], 'Fred' as [Item1]");
+            val.id.IsEqualTo(42);
+            val.name.IsEqualTo("Fred");
+        }
+
+    }
+}

Dapper/SqlMapper.cs

@@ -2232,11 +2232,42 @@ internal static IList<LiteralToken> GetLiteralTokens(string sql)
         {
             return CreateParamInfoGenerator(identity, checkForDuplicates, removeUnused, GetLiteralTokens(identity.sql));
         }
+        static bool IsValueTuple(Type type) => type != null && type.IsValueType() && type.FullName.StartsWith("System.ValueTuple`");
+
+        static List<IMemberMap> GetValueTupleMembers(Type type, string[] names)
+        {
+            var fields = type.GetFields(BindingFlags.Public | BindingFlags.Instance);
+            int keep = Math.Min(fields.Length, names.Length);
+            var result = new List<IMemberMap>(keep);
+            for(int i = 0; i < keep; i++)
+            {
+                FieldInfo field = null;
+                string name = "Item" + (i + 1).ToString(CultureInfo.InvariantCulture);
+                foreach(var test in fields)
+                {
+                    if(test.Name == name)
+                    {
+                        field = test;
+                        break;
+                    }
+                }
+                if (field != null)
+                {
+                    result.Add(new SimpleMemberMap(string.IsNullOrWhiteSpace(names[i]) ? name : names[i], field));
+                }
+            }
+            return result;
+        }
 
         internal static Action<IDbCommand, object> CreateParamInfoGenerator(Identity identity, bool checkForDuplicates, bool removeUnused, IList<LiteralToken> literals)
         {
             Type type = identity.parametersType;
 
+            if (IsValueTuple(type))
+            {
+                throw new NotSupportedException("ValueTuple should not be used for parameters - the language-level names are not available to use as parameter names, and it adds unnecessary boxing");
+            }
+
             bool filterParams = false;
             if (removeUnused && identity.commandType.GetValueOrDefault(CommandType.Text) == CommandType.Text)
             {
@@ -2924,7 +2955,6 @@ static LocalBuilder GetTempLocal(ILGenerator il, ref Dictionary<Type, LocalBuild
             ITypeMap typeMap = GetTypeMap(type);
 
             int index = startBound;
-
             ConstructorInfo specializedConstructor = null;
 
 #if !COREFX
@@ -3010,9 +3040,9 @@ static LocalBuilder GetTempLocal(ILGenerator il, ref Dictionary<Type, LocalBuild
                 il.Emit(OpCodes.Ldloc_1);// [target]
             }
 
-            var members = (specializedConstructor != null
+            var members = IsValueTuple(type) ? GetValueTupleMembers(type, names) :((specializedConstructor != null
                 ? names.Select(n => typeMap.GetConstructorParameter(specializedConstructor, n))
-                : names.Select(n => typeMap.GetMember(n))).ToList();
+                : names.Select(n => typeMap.GetMember(n))).ToList());
 
             // stack is now [target]