如何在.net 4中找到实体框架中字段的最大长度?

在ef 4.0中没有新的方法来访问属性的长度。您仍然需要浏览元数据-如图所示 in the accepted answer on the question you reference .

亚历克斯·詹姆斯

前英孚队员!

这种行为:

using System;
using System.Data.Objects;
using System.Data.Objects.DataClasses;
using System.Data.Metadata.Edm;
using System.Linq;
using System.Linq.Expressions;

namespace EfWidgets
{
    public class EntityWidgets
    {
        public static int GetMaxLength<TEntity>(ObjectContext oc, Expression<Func<TEntity, string>> property)
            where TEntity : EntityObject
        {
            var test = oc.MetadataWorkspace.GetItems(DataSpace.CSpace);

            if (test == null)
                return -1;

            Type entType = typeof(TEntity);
            string propertyName = ((MemberExpression)property.Body).Member.Name;

            var q = test
                .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                .SelectMany(meta => ((EntityType)meta).Properties
                .Where(p => p.Name == propertyName && p.TypeUsage.EdmType.Name == "String"));

            var queryResult = q.Where(p =>
            {
                var match = p.DeclaringType.Name == entType.Name;
                if (!match)
                    match = entType.Name == p.DeclaringType.Name;

                return match;

            })
                .Select(sel => sel.TypeUsage.Facets["MaxLength"].Value)
                .ToList();

            if (queryResult.Any())
            {
                int result = Convert.ToInt32(queryResult.First());
                return result;
            }
            return -1;
        }
    }
}