Is there a way to define implicit operator in C# that defines cast from Expression to selected class

Question

I'm trying to implement class that defines filter, to make it more explicit when it will be passed as argument to methods in my repository class (EF).

And I have a problem with definition of implicit operator from Expression to my class. Is it possible to make implementation for syntax near fd4 variable?

public class FilterDefinition<TEntity>
    where TEntity : class
{
    public FilterDefinition() { }

    public FilterDefinition(Expression<Func<TEntity, bool>> filter)
    {
        this.Filter = filter;
    }

    public virtual Expression<Func<TEntity, bool>> Filter { get; set; }

    public static implicit operator FilterDefinition<TEntity>(Expression<Func<TEntity, bool>> filter) => new FilterDefinition<TEntity>(filter);
}

public class SomeEntity
{
    public bool SomeBool { get; set; }
}

class Program
{
    static void Main()
    {
        FilterDefinition<SomeEntity> fd1 = new FilterDefinition<SomeEntity>(x => x.SomeBool);
        FilterDefinition<SomeEntity> fd2 = new FilterDefinition<SomeEntity> { Filter = x => x.SomeBool };
        FilterDefinition<SomeEntity> fd3 = (Expression<Func<SomeEntity, bool>>)(x => x.SomeBool);
        //FilterDefinition<SomeEntity> fd4 = x => x.SomeBool;

        Console.ReadKey();
    }
}

The purpose of FilterDefinition class was to pass as argument to generic repository of queries. And using inheritance define commonly used filters.

public class Repository<TEntity> : IRepository<TEntity>
    where TEntity : class
{
    private readonly DbSet<TEntity> dbSet;

    public Repository(DbContext context)
    {
        this.dbSet = context.Set<TEntity>();
    }

    public async Task<IEnumerable<TEntity>> GetAsync(
        FilterDefinition<TEntity> filter = null,
        OrderDefinition<TEntity> order = null,
        PagingDefinition paging = null)
    {
        return await new QueryBuilder<TEntity>(this.dbSet)
            .ApplyFilter(filter)
            .ApplyOrder(order)
            .ApplyPaging(paging)
            .ToQuery()
            .ToListAsync();
    }

Please post your code as _text_, not an image. And no, I don't think this can be done. This _might_ be an XY problem. What is `FilterDefinition` for anyway? — Sweeper, Feb 14 '20 at 07:19
You can't do it for the same reasons why you can't assign a lambda to a `var`. See https://stackoverflow.com/q/4965576/11683, https://stackoverflow.com/q/2687942/11683 and https://stackoverflow.com/q/411579/11683. — GSerg, Feb 14 '20 at 07:47

score 3 · Accepted Answer · answered Feb 14 '20 at 07:30

The short answer would be "not in this scenario", because syntactically, in fd4 you're not actually casting from an Expression[...] to your type. The compiler doesn't know that the lambda represents an Expression[...] until it knows what it is trying to do with the value; a lambda (x => x.SomeBool) can be interpreted as a wide range of both expression-tree types and delegate types (for anonymous methods), and the compiler here would not understand that you mean "treat the lambda as an Expression<Func<SomeEntity, bool>>, then use the implicit conversion operator to change the expression to a FilterDefinition". It simply can't (and doesn't) make that leap. You can get close, as with your fd3 example, but... a large part of me is wondering what FilterDefinition actually does here, because to an outside observer it doesn't really seem to add anything over the raw expression tree.

The purpose of FilterDefinition class was to pass as argument to generic repository of queries. And using inheritance define commonly used filters. — PiotrZ, Feb 14 '20 at 09:56

Is there a way to define implicit operator in C# that defines cast from Expression to selected class

1 Answers1