如何动态访问 DbSet 类型

Question

I'm using LINQ expressions in order to build select and where expressions dynamically.

The select method looks like this:

public Expression<Func<TSource, TTarget>> GetSelectExpression<TSource, TTarget>()
    {
        ParameterExpression l_pe = Expression.Parameter(Base_type, "source");  
...            
        Expression<Func<TSource, TTarget>> l_return = Expression.Lambda<Func<TSource, TTarget>>(l_select, new ParameterExpression[] { l_pe });
        l_return.Compile();

        return l_return;
    }

The Where method looks like the following:

public Expression<Func<TTarget, bool>> GetWhereExpression<TTarget>()
    {
        ParameterExpression l_pe = Expression.Parameter(Dto_type, "dto");

        ....

        Expression<Func<TTarget, bool>> l_return = Expression.Lambda<Func<TTarget, bool>>(l_or, new ParameterExpression[] { l_pe });
        l_return.Compile();

        return l_return;
    }

If I call the following statements, everything works fine:

var l_where = GetWhereExpression<CrudChangetaskDto>();
var l_select = GetSelectExpression<A_Changetask, CrudChangetaskDto>();               

IQueryable<CrudChangetaskDto> l_query = l_db.A_Changetask.Select(l_select).Where(l_where);
var lt_result = l_query.ToList();

Because of using the WHERE and SELECT methods more dynamically, I like to have a method returning an IQueryable like the following (PaltrConnect is the database context):

IQueryable<TTarget> GetSearchhelpvalue<TSource, TTarget>(PaltrConnect context)
    {   
        var l_dbset = context.Set(typeof(TSource));            

        IQueryable<TTarget> l_query = l_dbset.Select(GetSelectExpression<TSource,TTarget>()).Where(GetWhereExpression<TTarget>());
        return l_query;
    }

Unfortunately, this is syntactically not correct. I get the error message System.Linq.IQueryable cannot be converted into System.Linq.IQueryable.

Finally, I like to call GetSearchhelpvalue in the following way:

using (var l_db = new PaltrConnect)
  {
    IQueryable<CrudChangeTaskDto> l_query = GetSearchhelpvalue<A_Changetask, CrudChangetaskDto>(l_db);
    var lt_result = l_query.ToList();
  }

How can I access the DbSet of the type which corresponds to TSource or how can this be solved?

Yours Stephan

Answer 1

You are so close. Just instead of the non generic DbContext.Set(Type) use the generic DbContext.Set<TEntity>() overload like this

IQueryable<TTarget> GetSearchhelpvalue<TSource, TTarget>(PaltrConnect context)
    where TSource : class
{   
    return context.Set<TSource>()            
        .Select(GetSelectExpression<TSource,TTarget>())
        .Where(GetWhereExpression<TTarget>());
}

Note that it requires class constraint.