在谓词表达式中传递where子句

Question

I have the following predicate expression.

public IQueryable<T> Find(System.Linq.Expressions.Expression<Func<T, bool>> predicate)
    {
        return //boolean
    }

Here is where I am calling the method:

    public void TestMethod1()
    {
        author.AuthorName = authorName;

        using (var context = new AspBlogRepository<Author>())
        {
            if (context.Find(e => e.AuthorName == authorName))
            {
                //do nothing
            }
            context.Add(author);   
        }
    }

I get an error saying that I you cannot convert an IQueryable to bool. I just want to be able to use my predicate expression to see if the author is already in the database.

Any help would be much appreciated.

Thanks!

Answer 1

Error says it all. context.Find(e => e.AuthorName == authorName) is returning IQueryable<T> . If is expecting a bool

So your usage of if (context.Find(e => e.AuthorName == authorName)) is wrong. Change it to

if (context.Find(e => e.AuthorName == authorName).Any())
{

}

Answer 2

context.Find returns an IQueryable<T> . Your code could instead look like:

if (context.Find(e => e.AuthorName == authorName).Count() == 1)
{
    //do nothing
}

Answer 3

public IQueryable<T> Find(System.Linq.Expressions.Expression<Func<T, bool>> predicate)
    {
        return //boolean
    }

The problem is here, if you indeed return boolean. If this method is inside your context, you would first need to resolve Author and then filter the Authors in your repository.

If this is an EF context, and you have access to the EF context in your repository, you should be able to get the db set like this:

var entitySet = context.Set<T>();

Then, you can run your predicate against the entitySet returning the filtered results.