Question

I’m working with Entity Framework and Silverlight (RIA) and am looking at creating a function to expand on the CRUD to allow for the user to specify a specific column name, and then a matching value to pinpoint an exact record… The code would look something like this…

public IQueryable<Category> GetCategory(string theColumn, string theCriteria)
{
      return this.ObjectContext.Categories
                .Where(c => c.theColumn = theCriteria);
}

The similar working function to get ALL categories… (Created by building after associating a data model)

    public IQueryable<Category> GetCategories()
    {
        return this.ObjectContext.Categories;
    }

Thanks in advance!

Answer 1

I have built something similar to what you are looking to construct. I built around using Expression trees from System.Linq.Expressions. So I would suggest that you build your where predicate using an expression tree where the predicate has the signature of Expression<Func<T,bool>>.

Some pseudo code below where T is a WCF RIA entity:

public static class QueryHelper
{
  public static Expression<Func<T, bool>> ToPredicate<T> ( string propertyName, dynamic criteria )
    {
        ParameterExpression pe = Expression.Parameter( typeof( T ), "t" );
        Expression np = Expression.Property( pe, propertyName );
        ConstantExpression value = Expression.Constant( criteria );

        Expression e1 = Expression.Equal( np, value );

        var filter = Expression.Lambda<Func<T, bool>>( e1, pe );

        return filter;
    }
}

Which could then be used like:

var selector = QueryHelper.ToPredicate<Category>("theColumn", "hello");
return this.ObjectContext.Categories.Where(selector);     

LoadOperation<Employee> loader = context.Load( context.GetEmployeesQuery()
                                        .Where( selector ) );
loader.Completed += (op) =>
  {
    if ( !op.HasErrors)
    {
    }
  };

Or

var selector = QueryHelper.ToPredicate<Category>("theColumn", true);
return this.ObjectContext.Categories.Where(selector); 

One thing to note is the line Expression e1 = Expression.Equal( np, value ); in the function. You could extend this function to do >, <, =>, etc but you would need to check that the property type you were using supported the operator.

Hope this helps.