The following snippet indicates what I want:
public static class DblinqExtension
{
public static int MaxId<T>(this DbLinq.Data.Linq.Table<T> table) where T : class
{
var val = table.OrderByDescending(x => "Id").FirstOrDefault();
return Convert.开发者_开发问答ToInt32(val);
}
}
With DbMetal I've generated the mapping classes. Every table I have has the column Id (which is obviously an integer) and I want to know the MAX id.
Anyone any idea how I can get my snippet working??
Thanks!
I've found this article: OrderBy with a String keySelector
With that suggestion applied my code will become:
public static int MaxId<T>(this DbLinq.Data.Linq.Table<T> table) where T : class
{
var val = table.OrderByDescending(CreateSelectorExpression<T>("Id")).FirstOrDefault();
return Convert.ToInt32(val);
}
private static Expression<Func<T, Int32>> CreateSelectorExpression<T>(string propertyName) where T : class
{
var parameterExpression = Expression.Parameter(typeof(T));
return (Expression<Func<T, Int32>>)Expression.Lambda(
Expression.PropertyOrField(parameterExpression, propertyName),
parameterExpression
);
}
BUT now I get an error:
Value cannot be null. Parameter name: key
not sure but try
var Result = from x in table select x.Max ( x => x.Id );
EDIT - if you need to take Id as string into the expression:
var Result = from x in table select x.Max ( x => x.GetType().GetProperty ("Id").GetGetMethod().Invoke (x, null) );
EDIT - if you need to split up:
var Result1 = from x in table select x; var Result2 = Result1.Max(x => x.GetType().GetProperty("Id").GetGetMethod().Invoke(x, null));
EDIT - if "Id" is a field then you need this:
var Result1 = from x in table select x;
var Result2 = Result1.Max(x => x.GetType().GetField ( "Id" ).GetValue(x));
Alright, I've figured it out!!
public static int MaxId<T>(this DbLinq.Data.Linq.Table<T> table) where T : class
{
var param = Expression.Parameter(typeof(T), "p");
var body = Expression.PropertyOrField(param, "ID");
var lambda = Expression.Lambda<Func<T, int>>(body, param);
var val = table.OrderByDescending(lambda).FirstOrDefault();
return Convert.ToInt32(val.GetType().GetProperty("ID").GetGetMethod().Invoke(val, null));
}
Thanks a lot @Yahia for your hint with the GetProperty part!
精彩评论