allow Add to *replace* an existing template parameter in DynamicParameters,...

allow Add to *replace* an existing template parameter in DynamicParameters, allowing for out/return/etc usage

Dapper/SqlMapper.cs

@@ -2049,9 +2049,18 @@ void SqlMapper.IDynamicParameters.AddParameters(IDbCommand command, SqlMapper.Id
 
             foreach (var param in parameters.Values)
             {
-                var p = command.CreateParameter();
+                string name = Clean(param.Name);
+                bool add = !command.Parameters.Contains(name);
+                IDbDataParameter p;
+                if(add)
+                {
+                    p = command.CreateParameter();
+                    p.ParameterName = name;
+                } else
+                {
+                    p = (IDbDataParameter)command.Parameters[name];
+                }
                 var val = param.Value;
-                p.ParameterName = Clean(param.Name);
                 p.Value = val ?? DBNull.Value;
                 p.Direction = param.ParameterDirection;
                 var s = val as string;
@@ -2070,7 +2079,10 @@ void SqlMapper.IDynamicParameters.AddParameters(IDbCommand command, SqlMapper.Id
                 {
                     p.DbType = param.DbType.Value;
                 }
+                if (add)
+                {
                     command.Parameters.Add(p);
+                }
                 param.AttachedParam = p;
             }
         }

Tests/Tests.cs

@@ -1205,6 +1205,51 @@ public void TestFailInASaneWayWithWrongStructColumnTypes()
                 ex.Message.IsEqualTo("Error parsing column 0 (Value=1 - Int64)");
             }
         }
+
+
+        public void TestProcWithOutParameter()
+        {
+            connection.Execute(
+                @"CREATE PROCEDURE #TestProcWithOutParameter
+        @ID int output,
+        @Foo varchar(100),
+        @Bar int
+        AS
+        SET @ID = @Bar + LEN(@Foo)");
+            var obj = new
+            {
+                ID = 0,
+                Foo = "abc",
+                Bar = 4
+            };
+            var args = new DynamicParameters(obj);
+            args.Add("ID", 0, direction: ParameterDirection.Output);
+            connection.Execute("#TestProcWithOutParameter", args, commandType: CommandType.StoredProcedure);
+            args.Get<int>("ID").IsEqualTo(7);
+        }
+        public void TestProcWithOutAndReturnParameter()
+        {
+            connection.Execute(
+                @"CREATE PROCEDURE #TestProcWithOutAndReturnParameter
+        @ID int output,
+        @Foo varchar(100),
+        @Bar int
+        AS
+        SET @ID = @Bar + LEN(@Foo)
+        RETURN 42");
+            var obj = new
+            {
+                ID = 0,
+                Foo = "abc",
+                Bar = 4
+            };
+            var args = new DynamicParameters(obj);
+            args.Add("ID", 0, direction: ParameterDirection.Output);
+            args.Add("result", 0, direction: ParameterDirection.ReturnValue);
+            connection.Execute("#TestProcWithOutAndReturnParameter", args, commandType: CommandType.StoredProcedure);
+            args.Get<int>("ID").IsEqualTo(7);
+            args.Get<int>("result").IsEqualTo(42);
+        }
         struct CanHazInt
         {
             public int Value { get; set; }