Question

I am using .Net Framework 4, mvc 3 and entity framework (latest stable)

On an index page I want to be able to filter each column. The filter values are retrieved for each column by LINQ using the distinct() method. I have not found a good way to reuse the method that retrieves these values for each column – they are basically equal, the only thing differentiating them is the column name used, so having one method instead of several would be great.

public List<string> GetLevels()
{
    return _db.Logs.Select(l => l.Level).Distinct().ToList();
}

public List<string> GetOrders()
{
    return _db.Logs.Select(l => l.Order).Distinct().ToList();
}

How can I inject the lambda expression with the column name I want to retrieve data from? Something like this:

public List<string> GetFilterValues(string columName)
{
    return _db.Logs.Select(l => l.columnName).Distinct.ToList();
}

Answer 1

You can write a function which takes an expression selecting the property you want:

public List<T> GetLogProperties<T>(Expression<Func<Log, T>> selector)
{
    return _db.Logs.Select(selector).Distinct().ToList();
}

public List<string> GetLevels()
{
    return GetLogProperties(l => l.Level);
}

EDIT: If you want to create the projection from a string column, you will have to build the expression yourself:

public List<string> GetFilterValues(string columnName)
{
    var paramExpr = Expression.Parameter(typeof(Log), "l");
    var propExpr = Expression.Property(paramExpr, columnName);
    var lambdaExpr = Expression.Lambda<Func<Log, string>>(propExpr, paramExpr);

    return _db.Logs.Select(lambdaExpr).Distinct().ToList();
}

List<string> levels = GetFilterValues("Level");

Note that this will throw an exception if the property with the given name is not a string property.