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Answer 1

A:

See my answer to a similar question, which covers how to filter on a property selected from a dropdown list.

Bryan Watts 2009-12-18 04:43:22

Thanks that helps a lot, but is there a way to serialize/deserialize and Expression<>?

Eric Anastas 2009-12-18 05:24:30

If you want to enable raw text input from the user, you will have to do the parsing yourself. Translating from infix notation to expression trees will be a well-known and straightforward algorithm, but you'll still have to do it. A UI like your example is probably more feasible.

Bryan Watts 2009-12-18 05:37:07

Expressions are not inherently serializable. A viable design would be to store queries in whatever format makes the most sense for the criteria (i.e. a table to match what's in the dialog box), then translate that to an expression tree whenever you want to run it. This decouples your queries from their executable forms.

Bryan Watts 2009-12-18 05:39:47

Ahh ok makes sense. So I'll still use Linq to do the actual query but I could create a custom class that defines how the query is created that is serializable.

Eric Anastas 2009-12-18 13:07:53

Answer 2

+1 A:

This is not necessarily an ideal solution, but the MetaLinq project provides serializable wrappers for the LINQ expression classes.

Here is an example of how you can recreate and execute an expression after round-trip serializing it.

First we serialize an existing lamba expression into an xml document.

var originalExpr = EditableExpression.CreateEditableExpression<string, bool>(
    str => str.Length > 3);
var serializer = new XmlSerializer(originalExpr.GetType());
string xml;

using (var writer = new StringWriter())
{
    serializer.Serialize(writer, originalExpr);
    xml = writer.ToString();
}

Then we deserialize, unwrap and compile it back into a delegate, ready to be executed.

EditableExpression newExpr;

using (var reader = new StringReader(xml))
{
    newExpr = (EditableExpression) serializer.Deserialize(reader);
}

var expr = (Expression<Func<string, bool>>) newExpr.ToExpression();
var items = new[] {"one", "two", "three"};
var result = items.Count(expr.Compile());
Debug.Assert(result == 1);

This is what the serialized expression looks like.

<EditableLambdaExpression xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
  <NodeType>Lambda</NodeType>
  <TypeName>System.Func`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Boolean, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</TypeName>
  <Body xsi:type="EditableBinaryExpression">
    <NodeType>GreaterThan</NodeType>
    <TypeName>System.Boolean, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</TypeName>
    <Left xsi:type="EditableMemberExpression">
      <NodeType>MemberAccess</NodeType>
      <Expression xsi:type="EditableParameterExpression">
        <NodeType>Parameter</NodeType>
        <TypeName>System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</TypeName>
        <Name>str</Name>
      </Expression>
      <MemberName>System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Int32 Length</MemberName>
    </Left>
    <Right xsi:type="EditableConstantExpression">
      <NodeType>Constant</NodeType>
      <Value xsi:type="xsd:int">3</Value>
    </Right>
  </Body>
  <Parameters>
    <EditableExpression xsi:type="EditableParameterExpression">
      <NodeType>Parameter</NodeType>
      <TypeName>System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</TypeName>
      <Name>str</Name>
    </EditableExpression>
  </Parameters>
</EditableLambdaExpression>

Nathan Baulch 2009-12-18 10:12:42

Answer 3

A:

System.Linq.Expressions is unfortunately not serializable, not even in 4.0. You'll have to resort to write your own expression trees and visitors to generate queries from them if you have to cross appdomain boundaries. Otherwise, in box expression trees will be fine.

Apart from that, you may find dynamic linq suiting to your needs somewhat.

Tanveer Badar 2009-12-30 19:33:42

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