ParameterizedSqlExpression GetValue handle null when using parameterized statement

dialect provider base GetParamValue should call ToDbValue instead of FromDbValue
oracle dialect provider custom ToDbValue
fix handling of null in OracleEnumConverter

Author: yuinlin

src/ServiceStack.OrmLite.Oracle/Converters/OracleEnumConverter.cs

@@ -39,7 +39,7 @@ public override object ToDbValue(Type fieldType, object value)
                 return null;
             enumValue = enumValue.Trim('"');
             return enumValue == ""
-                ? "null"
+                ? null
                 : enumValue;
         }
     }

src/ServiceStack.OrmLite.Oracle/OracleOrmLiteDialectProvider.cs

@@ -7,6 +7,7 @@
 using System.Reflection;
 using System.Text;
 using System.Text.RegularExpressions;
+using ServiceStack.OrmLite.Converters;
 using ServiceStack.OrmLite.Oracle.Converters;
 
 namespace ServiceStack.OrmLite.Oracle
@@ -409,6 +410,40 @@ public override long InsertAndGetLastInsertId<T>(IDbCommand dbCmd)
         //    return value;
         //}
 
+        public override object ToDbValue(object value, Type type)
+        {
+            if (value == null || value is DBNull)
+                return null;
+
+            if (type.IsEnum)
+                return EnumConverter.ToDbValue(type, value);
+
+            if (type.IsRefType())
+                return ReferenceTypeConverter.ToDbValue(type, value);
+
+            IOrmLiteConverter converter = null;
+            try
+            {
+                if (Converters.TryGetValue(type, out converter))
+                {
+                    if (type == typeof(DateTimeOffset))
+                    {
+                        return converter.ToQuotedString(type, value);
+                    }
+
+                    return converter.ToDbValue(type, value);
+                }
+            }
+            catch (Exception ex)
+            {
+                Log.Error("Error in {0}.ToDbValue() value '{1}' and Type '{2}'"
+                    .Fmt(converter.GetType().Name, value != null ? value.GetType().Name : "undefined", type.Name), ex);
+                throw;
+            }
+
+            return base.ToDbValue(value, type);
+        }
+
         const string IsoDateFormat = "yyyy-MM-dd";
         const string IsoTimeFormat = "HH:mm:ss";
         const string IsoMillisecondFormat = "fffffff";

src/ServiceStack.OrmLite/Expressions/ParameterizedSqlExpression.cs

@@ -34,9 +34,11 @@ protected virtual void ConvertToPlaceholderAndParameter(ref object right)
 
         public override object GetValue(object value, Type type)
         {
-            return ((!OrmLiteConfig.UseParameterizeSqlExpressions) || SkipParameterizationForThisExpression)
-                ? DialectProvider.GetQuotedValue(value, type)
-                : DialectProvider.GetParamValue(value, type);
+            if ((!OrmLiteConfig.UseParameterizeSqlExpressions) || SkipParameterizationForThisExpression)
+                return DialectProvider.GetQuotedValue(value, type);
+
+            var paramValue = DialectProvider.GetParamValue(value, type);
+            return paramValue ?? "null";
         }
 
         protected override void VisitFilter(string operand, object originalLeft, object originalRight, ref object left, ref object right)

src/ServiceStack.OrmLite/OrmLiteDialectProviderBase.cs

@@ -353,7 +353,7 @@ public virtual object ToDbValue(object value, Type type)
             }
             catch (Exception ex)
             {
-                Log.Error("Error in {0}.FromDbValue() value '{1}' and Type '{2}'"
+                Log.Error("Error in {0}.ToDbValue() value '{1}' and Type '{2}'"
                     .Fmt(converter.GetType().Name, value != null ? value.GetType().Name : "undefined", type.Name), ex);
                 throw;
             }
@@ -365,7 +365,7 @@ public virtual object ToDbValue(object value, Type type)
             }
             catch (Exception)
             {
-                Log.ErrorFormat("Error FromDbValue trying to convert {0} into {1}", value, type.Name);
+                Log.ErrorFormat("Error ToDbValue trying to convert {0} into {1}", value, type.Name);
                 throw;
             }
         }
@@ -1406,7 +1406,7 @@ public virtual string GetQuotedValue(object value, Type fieldType)
 
         public virtual object GetParamValue(object value, Type fieldType)
         {
-            return FromDbValue(value, fieldType);
+            return ToDbValue(value, fieldType);
         }
 
         public virtual string EscapeWildcards(string value)