Creating a lambda expression at runtime

Question

I have a repository class which has a GetAsQueryable method defined as follows:

public class Repository<TEntity> : IDisposable, IRepository<TEntity> where TEntity : class
{
    internal DbSet<TEntity> _DbSet;

    public virtual IQueryable<TEntity> GetAsQueryable(
         Expression<Func<TEntity, bool>> filter = null,
         Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null,
         string includeProperties = "")
    {
        IQueryable<TEntity> query = _DbSet;

        if (filter != null)
        {
            query = query.Where(filter);
        }

        foreach (var includeProperty in includeProperties.Split
            (new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries))
        {
            query = query.Include(includeProperty);
        }

        if (orderBy != null)
        {
            return orderBy(query);
        }
        else
        {
            return query;
        }
    }
}

In my calling code I typically do operations such as:

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();
var results = tracking.Where(t => t.Status_Code_Id == 15).ToList();

This works great; however now I want to be able to build the lambda sent into the .Where at runtime. I have tried building an expression tree as follows:

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();
var results = tracking.Where(t => t.Status_Code_Id == 15).ToList();

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();

ParameterExpression pe = Expression.Parameter(typeof (int), "Status_Code_Id");
LambdaExpression lambda = Expression.Lambda(Expression.Equal(pe, Expression.Constant(15)));
MethodCallExpression whereExpression = Expression.Call(
    typeof(Queryable),
    "Where",
    new Type[] { tracking.ElementType },
    tracking.Expression,
    lambda);

However, this yields the following exception:

No generic method 'Where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

Why can it not find the Where method on my instance of Tracking?

MarcinJuraszek · Accepted Answer · 2015-07-14 22:31:58Z

8

If you want to build t => t.Status_Code_Id == 15 dynamically you need don't need to call Where using Expression. Just create Expression<Func<TEntity, bool>> and pass it to Where:

ParameterExpression t = Expression.Parameter(typeof(Tracking), "t");
Expression statusCode = Expression.Property(t, "Status_Code_Id");
Expression comparison = Expression.Equal(statusCode, Expression.Constant(15));
Expression<Func<Tracking, bool>> lambda
    = Expression.Lambda<Func<Tracking, bool>>(comparison, t);

var results = tracking.Where(lambda).ToList();

answered Jul 14, 2015 at 22:31

MarcinJuraszek

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1 Comment

codechurn Over a year ago

I was missing the Expression.Parameter(typeof(Tracking), "t"); piece. Being able to build the lambda to pass into the .Where method is exactly what I was looking to do! @MarcinJuraszek Dziękuję bardzo!

StriplingWarrior · Accepted Answer · 2015-07-14 22:32:05Z

0

Remember that .Where() has a generic signature. You don't have to provide explicit generic parameters to it when you use a lambda expression because the compiler can infer the types for you, but when you're using reflection you'll need to create a generic signature to call based on the types that you're dealing with.

answered Jul 14, 2015 at 22:32

StriplingWarrior

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