Question

我使用EF4和我发现的一段代码从这样的实体中获取MaxLength值：

public static int? GetMaxLength(string entityTypeName, string columnName)
        {
            int? result = null;
            using (fooEntities context = new fooEntities())
            {
                Type entType = Type.GetType(entityTypeName);
                var q = from meta in context.MetadataWorkspace.GetItems(DataSpace.CSpace)
                                  .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                        from p in (meta as EntityType).Properties
                        .Where(p => p.Name == columnName
                                    && p.TypeUsage.EdmType.Name == "String")
                        select p;

                var queryResult = q.Where(p =>
                {
                    bool match = p.DeclaringType.Name == entityTypeName;
                    if (!match && entType != null)
                    {
                        match = entType.Name == p.DeclaringType.Name;
                    }

                    return match;

                }).Select(sel => sel.TypeUsage.Facets["MaxLength"].Value);
                if (queryResult.Any())
                {
                    result = Convert.ToInt32(queryResult.First());
                }

                return result;
            }
        }

但是，我升级到EF5，我知道收到此错误消息：

...fooEntities'  does not contain a definition for 'MetadataWorkspace' and no
extension method 'MetadataWorkspace' accepting a first argument of type
'...fooEntities' could be found (are you missing a using directive or an assembly
 reference?)

从EF5获取元数据的最佳方法是什么？

Answer 1

这是一个非常方便的代码。我重构了一下，它非常有用，我想我会在这里发布它。

public static int? GetMaxLength<T>(Expression<Func<T, string>> column)
    {
        int? result = null;
        using (var context = new EfContext())
        {
            var entType = typeof(T);
            var columnName = ((MemberExpression) column.Body).Member.Name;

            var objectContext = ((IObjectContextAdapter) context).ObjectContext;
            var test = objectContext.MetadataWorkspace.GetItems(DataSpace.CSpace);

            if(test == null)
                return null;

            var q = test
                .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                .SelectMany(meta => ((EntityType) meta).Properties
                .Where(p => p.Name == columnName && 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())
                result = Convert.ToInt32(queryResult.First());

            return result;
        }
    }

你可以这样称呼：

GetMaxLength<Customer>(x => x.CustomerName);

假设您在DbContext中定义了一个类型为Customer的DbSet，它具有CustomerName属性，并具有已定义的MaxLength。

这对于创建模型属性非常有用，这些属性将文本框的maxlength设置为数据库中字段的最大长度，始终确保两者相同。

Answer 2

这意味着您不仅升级了EF，而且还更改了API。有两个API - 核心ObjectContext API和简化的DbContext API。您的代码依赖于ObjectContext API（EF4中唯一可用的API），但EF5使用DbContext API（自EF4.1起添加在单独的EntityFramework.dll程序集中）。如果您想使用新的EF功能和以前的代码，您应该只升级到.NET 4.5。

如果您还想使用新的API，则必须更新现有的许多代码，但仍然可以从ObjectContext获取DbContext并使您的方法再次运行。您只需使用此代码段：

var objectContext = ((IObjectContextAdapter)context).ObjectContext;

并在代码中使用objectContext代替context。

Answer 3

我将mccow002的示例重构为一个准备好复制粘贴的Extension方法类：

using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Metadata.Edm;

public static class DbContextExtensions
{

    // get MaxLength as an extension method to the DbContext
    public static int? GetMaxLength<T>(this DbContext context, Expression<Func<T, string>> column)
    {
        return (int?)context.GetFacets<T>(column)["MaxLength"].Value;
    }

    // get MaxLength as an extension method to the Facets (I think the extension belongs here)
    public static int? GetMaxLength(this ReadOnlyMetadataCollection<Facet> facets)
    {
        return (int?)facets["MaxLength"].Value;            
    }

    // just for fun: get all the facet values as a Dictionary 
    public static Dictionary<string,object> AsDictionary(this ReadOnlyMetadataCollection<Facet> facets) {
        return facets.ToDictionary(o=>o.Name,o=>o.Value);
    }


    public static ReadOnlyMetadataCollection<Facet> GetFacets<T>(this DbContext context, Expression<Func<T, string>> column)
    {
        ReadOnlyMetadataCollection<Facet> result = null;

        var entType = typeof(T);
        var columnName = ((MemberExpression)column.Body).Member.Name;

        var objectContext = ((IObjectContextAdapter)context).ObjectContext;
        var test = objectContext.MetadataWorkspace.GetItems(DataSpace.CSpace);

        if (test == null)
            return null;

        var q = test
            .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
            .SelectMany(meta => ((EntityType)meta).Properties
            .Where(p => p.Name == columnName && 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)
            .FirstOrDefault();

        result = queryResult.TypeUsage.Facets;

        return result;

    }

}

Answer 4

我有类似的问题，解决方案在这里;

  MyDBEntities ctx = new MyDBEntities();
        var objectContext = ((IObjectContextAdapter)ctx).ObjectContext;

        var cols = from meta in objectContext.MetadataWorkspace.GetItems(DataSpace.CSpace)
                   .Where(m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType)
                   from p in (meta as EntityType).Properties
                      .Where(p => p.DeclaringType.Name == "TableName")
                   select new
                   {
                       PropertyName = p.Name                          
                   };

实体框架5 MaxLength

4 个答案: