Select(" new (...)") into a ListT> (or any other enumerable collection of T>) using System.LINQ.Dynamic.

dynamic-linq linq

Question

Say I have a DataTable with four columns, Company (string), Fund (string), State (string), Value(double):

    table1.Rows.Add("Company 1","Fund 1","NY",100));
    table1.Rows.Add("Company 2","Fund 1","CA",200));
    table1.Rows.Add("Company 3","Fund 1","FL",300));
    table1.Rows.Add("Company 4","Fund 2","CA",400));
    table1.Rows.Add("Company 5","Fund 1","NY",500));
    table1.Rows.Add("Company 6","Fund 2","CA",600));
    table1.Rows.Add("Company 7","Fund 3","FL",700));

I want to use System.LINQ.Dynamic to build a dynamic query which groups on either Company, Fund, or State, and then selects my group by criteria as the first column, and sum(value):

string groupbyvalue="Fund";
var q1= table1.AsEnumerable().AsQueryable()
              .GroupBy(groupbyvalue,"it")
              .Select("new ("+groupbyvalue+" as Group, Sum(Value) as TotalValue)");

In the above query, the selected groupbyvalue (Group) will always be a string, and the sum will always be a double, so I want to be able to cast into something like a List, where Result is an object with properties Group (string) and TotalValue (double).

I'm having a lot of trouble with this, can anyone shed some light?

1
20
4/30/2014 1:08:04 PM

Popular Answer

First, you'll access the current grouped value as Key in your Select clause:

.Select("new (Key as Group, Sum(Value) as TotalValue)");

That should make your query work. The harder question is how to turn the returned objects, which will have a dynamically generated type that inherits from DynamicClass, into a static type.

Option 1: Use reflection to access the dynamic object's Group and TotalValue properties.

Option 2: Use compiled expression trees for lightweight code generation to access the Group and TotalValue properties.

Option 3: Modify the Dynamic library to support a strongly-typed result. This turns out to be rather simple:

  1. In ExpressionParser.Parse(), capture the type argument in a private field:

    private Type newResultType;
    public Expression Parse(Type resultType)
    {
        newResultType = resultType;
        int exprPos = token.pos;
        // ...
    
  2. Near the end of ExpressionParser.ParseNew(), we'll try to use newResultType before defaulting to a dynamic type:

    Expression ParseNew()
    {
        // ...
        NextToken();
        Type type = newResultType ?? DynamicExpression.CreateClass(properties);
        MemberBinding[] bindings = new MemberBinding[properties.Count];
        for (int i = 0; i < bindings.Length; i++)
            bindings[i] = Expression.Bind(type.GetProperty(properties[i].Name), expressions[i]);
        return Expression.MemberInit(Expression.New(type), bindings);
    }
    
  3. Finally, we need a strongly typed version of Select():

    public static IQueryable<TResult> Select<TResult>(this IQueryable source, string selector, params object[] values)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (selector == null) throw new ArgumentNullException("selector");
        LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, typeof(TResult), selector, values);
        return source.Provider.CreateQuery<TResult>(
            Expression.Call(
                typeof(Queryable), "Select",
                new Type[] { source.ElementType, typeof(TResult) },
                source.Expression, Expression.Quote(lambda)));
    }
    

    The only changes from the original Select() are places we reference TResult.

Now we just need a named type to return:

    public class Result
    {
        public string Group { get; set; }
        public double TotalValue { get; set; }
    }

And your updated query will look like this:

    IQueryable<Result> res = table1.AsQueryable()
        .GroupBy(groupbyvalue, "it")
        .Select<Result>("new (Key as Group, Sum(Value) as TotalValue)");
42
9/23/2009 8:33:32 PM


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