Dynamic where clause in lambda

Question

I am using entity framework, and I need to create a dynamic expressions like:

var sel = Expression.Lambda<Func<TEntity, bool>>(propertyAccess, parameter);
var compiledSel = sel.Compile();
// sel = x.Name
// filter.Value = "John"
Repository.GetData.Where(item => compiledSel(item) != null && compiledSel(item).ToLower().StartsWith(filter.Value.ToString().ToLower()))

The above works with IQueriable, but I need it to work with entity framework.

That means I need to parse

item => compiledSel(item) != null && compiledSel(item).ToLower().StartsWith(filter.Value.ToString().ToLower())

to e.g.

x => x.Name != null && x.Name.StartsWith("John")

The reason I am doing this is because I have multiple entities I want to be able to filter dynamically.

Any suggestions?

Edit:

The query itself against EF is run here:

private IList<TEntity> GetCollection(Expression<Func<TEntity, bool>> where, Expression<Func<TEntity, object>>[] includes)
{
    return DbSet 
     .Where(where)
     .ApplyIncludes(includes)
     .ToList();
}

When I run the query now the data where clause is Param_0 => (((Invoke(value(.... and I get The LINQ expression node type 'Invoke' is not supported in LINQ to Entities. error

how about adding an interface to your entity? e.g. class MyEntity:INameable with property string Name and then you can work with Func<INameable, bool> dynamically — Matthias Burger
– Matthias Burger, Commented Aug 29, 2016 at 11:56
What does propertyAccess variable contain? String property accessor? So sel is actually Expression<Func<TEntity, string>> rather than Expression<Func<TEntity, bool>> as in the example? — Ivan Stoev
– Ivan Stoev, Commented Aug 29, 2016 at 11:59
The problem with adding INameble, is that I have a lot of entities that are different, so "x.Name" is just an example, it could be a quite a big number of other things — h3li0s
– h3li0s, Commented Aug 29, 2016 at 12:01
the propperyAccess contains a MemberExpression in this example {x.Name} — h3li0s
– h3li0s, Commented Aug 29, 2016 at 12:05
I'm pretty sure you are using the overload IEnumerable<T>.Where(Func<T, bool>) overload which accepts an Func<T, bool> instead of IQueryable<T>.Where(Expression<Func<T, bool>>) which accepts an expression. Your problem is not that 'it works with IQueryable, but not with EF`. — Maarten
– Maarten, Commented Aug 29, 2016 at 12:12

Ivan Stoev · Accepted Answer · 2016-08-29 12:29:40Z

First off, if the propertyAccess is accessor to a string property, the following

var sel = Expression.Lambda<Func<TEntity, bool>>(propertyAccess, parameter);

should be

var sel = Expression.Lambda<Func<TEntity, string>>(propertyAccess, parameter);

Second, the Compile does not work inside the EF expressions. You can build the whole predicate expression manually using the methods of the Expression class, but that's relatively hard. What I could suggest you is to use a "prototype" expression and simple parameter replacer like this:

var selector = Expression.Lambda<Func<TEntity, string>>(propertyAccess, parameter);

var value = filter.Value.ToString().ToLower();

Expression<Func<string, bool>> prototype = 
    item => item != null && item.ToLower().StartsWith(value);

var predicate = Expression.Lambda<Func<T, bool>>(
    prototype.Body.ReplaceParameter(prototype.Parameters[0], selector.Body), 
    selector.Parameters[0]);

Here is the code of the used helper method:

public static class ExpressionUtils
{
    public static Expression ReplaceParameter(this Expression expression, ParameterExpression source, Expression target)
    {
        return new ParameterReplacer { Source = source, Target = target }.Visit(expression);
    }

    class ParameterReplacer : ExpressionVisitor
    {
        public ParameterExpression Source;
        public Expression Target;
        protected override Expression VisitParameter(ParameterExpression node)
        {
            return node == Source ? Target : base.VisitParameter(node);
        }
    }
}

dchanko · Accepted Answer · 2016-08-29 12:29:30Z

1

Dynamic Linq might be an option for you:

Repository.People.Where("Name != null && Name.StartsWith(\"John\")")

https://www.nuget.org/packages/System.Linq.Dynamic

answered Aug 29, 2016 at 12:29

dchanko

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