Cannot convert from Expression< Func< Entity, bool> > to Func< Entity, bool>

Question

I have this function in my code:

public virtual bool Exists<ENTITY>(Expression<Func<ENTITY, bool>> expr)
{
    return this._dbSet.Count(expr) > 0;
}

I get the error

Cannot convert System.Linq.Expressions.Expression< System.Func< ENTITY, bool>> to System.Func< ENTITY, bool>

now if I change function to this:

public virtual bool Exists<ENTITY>(Expression<Func<ENTITY, bool>> expr)
{
    var tmp = expr.Compile();
    return this._dbSet.Count(tmp) > 0;
}

I get another error:

Cannot convert System.Func< ENTITY, bool> to System.Linq.Expressions.Expression< System.Func< ENTITY, bool> >

What am I doing wrong or is the compiler got crazy? Im using VS2017 C# 7.1

Answer 1

You didn't post your full code so I'm guessing here:

You class already is generic with a parameter ENTITY . So drop the <ENTITY> of Exists<ENTITY> because that means your method is generic and different from your class . But it must be the same generic parameter as the class and your DbSet so just use the class' generic parameter.

class Repository<ENTITY>
{
    public virtual bool Exists(Expression<Func<ENTITY, bool>> expr)
    {
        return this._dbSet.Count(expr) > 0;
    }
}

---

Sidenote: an EXISTS would probably be an .Any() , not .Count() > 0 .

Answer 2

Assuming that you want to count the entities in _dbSet that fulfill the predicate - in order to check if there is any element at all , after compiling the expression tree,

var tmp = expr.Compile();

you should add this: return this._dbSet.Where(entity => tmp(entity).Count() > 0;

Update

As already nvoigt has pointed out in his answer, it would be more logical to use the Any method:

return this._dbSet.Any(entity => tmp(entity));