如何使用表达式树对IQueryable对象动态排序？

Question

Currently I have a custom datagridview where the user can browse true it's pages with buttons like next, previous, first and last.

In the generic part of the code I have the following IQueryable object: IQueryable iqRetVal = iqData; . Its contents is dynamically filled by other forms and it could contain for example users and the next time companies (So there is no way of knowing beforehand what will be in it.).

This is the code for iqData :

IQueryable iqData = null;
public IQueryable Data //this is how iqData is filled.
{
    get { return iqData; }
    set
    {
        iqData = value;
        RefreshDataGridView();
    }
}

Now I know what the user previously clicked on based on 2 variables:

private SortOrder sortOrder; //The sort order like ascending or descending
private DataGridViewColumn dataGridViewColumn; the column that was sorted on.

This is the error message that my code gives:

No generic method OrderByDescending in type System.Linq.Queryable is compateble with the given typearguments and arguments. No typearguments must ge given if the method is not generic.

The code itself:

try
{
    if (dataGridViewColumn != null)
    {
        var propName = dataGridViewColumn.DataPropertyName;
        ParameterExpression param = null;

        foreach (var item in iqRetVal)
        {
            if (item != null)
            {
                param = Expression.Parameter(item.GetType(), string.Empty);
                break;
            }
        }
        MemberExpression property = Expression.PropertyOrField(param, propName);
        LambdaExpression sort = Expression.Lambda(property, param);

        if (sortOrder == SortOrder.Descending)
        {
            iqRetVal = iqRetVal.Provider.CreateQuery(Expression.Call(typeof(Queryable), "OrderBy", new Type[] { iqRetVal.ElementType }, iqRetVal.Expression, Expression.Quote(sort)));
        }
        else
        {
            iqRetVal = iqRetVal.Provider.CreateQuery(Expression.Call(typeof(Queryable), "OrderByDescending", new Type[] { iqRetVal.ElementType }, iqRetVal.Expression, Expression.Quote(sort)));
        }

    }
    else { }
}
catch(Exception ex)
{
    MessageBox.Show(ex.Message);
    System.Diagnostics.Debug.WriteLine(ex.Message);
}

What exactly am I missing to make my code work? Because I have searched for a long time but it seems like most answers don't offer the answer I'm looking for.

Answer 1

The method call causing problems is this:

Expression.Call(typeof(Queryable), "OrderBy", new Type[] { iqRetVal.ElementType }, iqRetVal.Expression, Expression.Quote(sort));

Looking at the documentation for this overload of CreateQuery we can see that the first parameter is the type that contains the static method, the second is the method name, the third is the generic parameters needed by that method and the subsequent items are the parameters to the method.

The OrderBy method does exist on Queryable so our first two paramters are good. If we look at this method though then we see that it has two generic parameters: OrderBy<TSource,TKey> . Currently we are only passing one type. This makes sense of the error message which says the type arguments are incorrect.

So our first type is the type of the Queryable which we are correctly getting as iqRetVal.ElementType . The second is the type of key that our sort selector returns. This can be found using sort.ReturnType .

So this would then make the method call look like this:

Expression.Call(typeof(Queryable), "OrderBy", new Type[] { iqRetVal.ElementType, sort.ReturnType }, iqRetVal.Expression, Expression.Quote(sort))