fixed #47 Get now supports DBNull properly

Dapper/SqlMapper.cs

@@ -1586,7 +1586,16 @@ void SqlMapper.IDynamicParameters.AddParameters(IDbCommand command)
 
         public T Get<T>(string name)
         {
-            return (T)parameters[Clean(name)].AttachedParam.Value;
+            var val = parameters[Clean(name)].AttachedParam.Value;
+            if (val == DBNull.Value)
+            {
+                if (default(T) != null)
+                {
+                    throw new ApplicationException("Attempting to cast a DBNull to a non nullable type!");
+                }
+                return default(T);
+            }
+            return (T)val;
         }
     }
     public sealed class DbString

Tests/Tests.cs

@@ -1026,5 +1026,15 @@ public void TestMultiMapThreeTypesWithGridReader()
 
             connection.Execute("drop table #Users drop table #Posts drop table #Comments");
         }
+
+        public void TestDynamicParamNullSupport()
+        {
+            var p = new DynamicParameters();
+
+            p.Add("@b", dbType: DbType.Int32, direction: ParameterDirection.Output);
+            connection.Execute("select @b = null",p);
+
+            p.Get<int?>("@b").IsNull();
+        }
     }
 }