Using dynamic objects in codedom created LINQ queries

Question

I tried to use expandoobjects in LINQ queries to have the ability to query against properties that are created during runtime, for example the headers from a csv file. It all worked fine if typing the LINQ query direct in the code as in the example:

// initialize testdata
List<ExpandoObject> hans = new List<ExpandoObject>();
string[] names = {"Apfel", "Birne", "Banane", "Orange"};
int[] ids = { 1, 2, 3, 4 };
for (int i = 0; i < 4; i++)
{
   dynamic horst = new ExpandoObject();
   ((IDictionary<string, object>)horst).Add("Fruit", names[i]);
   ((IDictionary<string, object>)horst).Add("ID", ids[i]);
   hans.Add(horst);
}

// try some LINQ queries, both are working as intended
var klaus = from dynamic x in hans where x.ID < 3 select x;
//var klaus = hans.Where(x => x.ID < 3).Select(x => x);

Then i tried to read the query from the commandline and create a dynamic LINQ query using a slighty modified version of the evaluant linq compiler.

string expression = System.Console.ReadLine();
LinqCompiler lc = new LinqCompiler(expression);
lc.AddSource<ExpandoObject>("hans", hans);
IEnumerable<ExpandoObject> klaus = (IEnumerable<ExpandoObject>)lc.Evaluate();

As long as as i don´t use WHERE or ORDER BY statements, everything is fine, but if any WHERE or ORDER BY is included in the query, i get an error when compiling the codedom code in the linq compiler: CS1963: An expression tree may not contain a dynamic operation.

The code for the query is created using the following line:

doRequestMethod.Statements.Add(new CodeMethodReturnStatement(new CodeSnippetExpression(Query)));

I suppose that the codedom compiler is building the expression tree in some way different to the way a direct typed in LINQ query is parsed. Any idea to get this to work would be appretiated, including other ideas to dynamically create queries for runtime-generated objects.

score 2 · Accepted Answer · answered Apr 11 '12 at 17:35

To get the error you're getting, I had to fix the LINQ Compiler to support dynamic, by telling it to use C# 4.0 and add a reference to Microsoft.CSharp.dll, so I assume you have done the same.

The problem is a source in LINQ compiler can be any collection, including IQueryable<T>. And if IQueryable<T> is supposed to work correctly, you actually need to treat it as IQueryable<T>, not IEnumerable<T>. The way LINQ compiler solves this is that it treats any source as IQuerybale<T>by using the AsQueryable() extension method.

This means the generated code looks like this:

public object DoRequest(System.Linq.IQueryable<System.Dynamic.ExpandoObject> hans) {
    return from dynamic x in hans where x.ID < 3 select x;
}

The problem with this code is that tries to use IQuerybale<T> versions of LINQ methods, that use Expressions. And, as the error message tells you, Expressions don't support dynamic.

I think the easiest way to fix this is to modify LINQ Compiler to use IEnumerable<T>, instead of IQuerybale<T> by changing the AddSource() into:

public void AddSource<T>(string name, IEnumerable<T> source) 
{
    this.sources.Add(new SourceDescription(name, typeof(IEnumerable<T>), source));
}

Of course, this means it won't work well for database queries, but you can't make database queries work with dynamic anyway.

Thank you, this made it work if changing the call of the DoRequest method, too. It is based on a List list and calls toQueryabale() on your source. `List list = new List(); foreach (SourceDescription source in this.sources) { list.Add(source.Instance); }` — Robin G, Apr 11 '12 at 21:49

Using dynamic objects in codedom created LINQ queries

1 Answers1