Tom Deloford Tom Deloford - 1 month ago 13
C# Question

How to create LINQ Expression Tree to select an anonymous type

I would like to generate the following select statement dynamically using expression trees:

var v = from c in Countries
where c.City == "London"
select new {c.Name, c.Population};


I have worked out how to generate

var v = from c in Countries
where c.City == "London"
select new {c.Name};


but I cannot seem to find a constructor/overload that will let me specify multiple properties in my select lambda.

Answer

This can be done, as mentioned, with the help of Reflection Emit and a helper class I've included below. The code below is a work in progress, so take it for what it's worth... 'it works on my box'. The SelectDynamic method class should be tossed in a static extension method class.

As expected, you won't get any Intellisense since the type isn't created until runtime. Works good on late-bound data controls.

public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
{
    Dictionary<string, PropertyInfo> sourceProperties = fieldNames.ToDictionary(name => name, name => source.ElementType.GetProperty(name));
    Type dynamicType = LinqRuntimeTypeBuilder.GetDynamicType(sourceProperties.Values);

    ParameterExpression sourceItem = Expression.Parameter(source.ElementType, "t");
    IEnumerable<MemberBinding> bindings = dynamicType.GetFields().Select(p => Expression.Bind(p, Expression.Property(sourceItem, sourceProperties[p.Name]))).OfType<MemberBinding>();

    Expression selector = Expression.Lambda(Expression.MemberInit(
        Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), sourceItem);

    return source.Provider.CreateQuery(Expression.Call(typeof(Queryable), "Select", new Type[] { source.ElementType, dynamicType },
                 Expression.Constant(source), selector));
}



public static class LinqRuntimeTypeBuilder
{
    private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
    private static AssemblyName assemblyName = new AssemblyName() { Name = "DynamicLinqTypes" };
    private static ModuleBuilder moduleBuilder = null;
    private static Dictionary<string, Type> builtTypes = new Dictionary<string, Type>();

    static LinqRuntimeTypeBuilder()
    {
        moduleBuilder = Thread.GetDomain().DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.Run).DefineDynamicModule(assemblyName.Name);
    }

    private static string GetTypeKey(Dictionary<string, Type> fields)
    {
        //TODO: optimize the type caching -- if fields are simply reordered, that doesn't mean that they're actually different types, so this needs to be smarter
        string key = string.Empty;
        foreach (var field in fields)
            key += field.Key + ";" + field.Value.Name + ";";

        return key;
    }

    public static Type GetDynamicType(Dictionary<string, Type> fields)
    {
        if (null == fields)
            throw new ArgumentNullException("fields");
        if (0 == fields.Count)
            throw new ArgumentOutOfRangeException("fields", "fields must have at least 1 field definition");

        try
        {
            Monitor.Enter(builtTypes);
            string className = GetTypeKey(fields);

            if (builtTypes.ContainsKey(className))
                return builtTypes[className];

            TypeBuilder typeBuilder = moduleBuilder.DefineType(className, TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.Serializable);

            foreach (var field in fields)                    
                typeBuilder.DefineField(field.Key, field.Value, FieldAttributes.Public);

            builtTypes[className] = typeBuilder.CreateType();

            return builtTypes[className];
        }
        catch (Exception ex)
        {
            log.Error(ex);
        }
        finally
        {
            Monitor.Exit(builtTypes);
        }

        return null;
    }


    private static string GetTypeKey(IEnumerable<PropertyInfo> fields)
    {
        return GetTypeKey(fields.ToDictionary(f => f.Name, f => f.PropertyType));
    }

    public static Type GetDynamicType(IEnumerable<PropertyInfo> fields)
    {
        return GetDynamicType(fields.ToDictionary(f => f.Name, f => f.PropertyType));
    }
}