Question

我想使用LINQ查询返回在数据库中存储为整数的枚举的字符串值。

我尝试过：

  return (from a in context.Tasks
                    select new TaskSearch
                    {
                        TaskID = a.TaskID,
                        TaskTypeName = Enum.GetName(typeof(TaskTypeEnum), a.TaskType)
                    }).ToList();

我使用的是asp.net mvc。

异常：类型＆＃39; System.NotSupportedException＆＃39;的例外情况发生在EntityFramework.SqlServer.dll中但未在用户代码中处理

附加信息：LINQ to Entities无法识别方法＆＃39; System.String GetName（System.Type，System.Object）＆＃39;方法，并且此方法无法转换为商店表达式。

Answer 1

您需要实现查询（查询必须能够转换为sql语句，但Enum.GetName()无法转换为sql）

尝试

((from a in context.Tasks select a).AsEnumerable().Select(t => new TaskSearch
{
  TaskID = t.TaskID,
  TaskTypeName = Enum.GetName(typeof(TaskTypeEnum), t.TaskType)

}).ToList());

Answer 2

我定义了一组这样的Enum扩展，我发现它很有用。不幸的是，类型约束不能超出struct，因此您必须从外部确保仅在枚举上调用方法。

/// <summary>Type-safe extension methods for parsing Enums.</summary>
public static partial class EnumExtensions{
  #region Enum Parsing utilities
  /// <summary>Typesafe wrapper for <c>Enum.GetValues(typeof(TEnum).</c></summary>
  public static ReadOnlyCollection<TEnum> EnumGetValues<TEnum>() {
    return new ReadOnlyCollection<TEnum>((TEnum[])(Enum.GetValues(typeof(TEnum))));
  }

  /// <summary>TODO</summary>
 [SuppressMessage("Microsoft.Design", "CA1004:GenericMethodsShouldProvideTypeParameter")]
  public static ReadOnlyCollection<string> EnumGetNames<TEnum>() where TEnum : struct {
    return new ReadOnlyCollection<string>((string[])(Enum.GetNames(typeof(TEnum))));
  }

  /// <summary>Typesafe wrapper for <c>Enum.ParseEnum()</c> that automatically checks 
  /// constants for membership in the <c>enum</c>.</summary>
  public static TEnum ParseEnum<TEnum>(string value) where TEnum : struct {
    return ParseEnum<TEnum>(value,true);
  }

  /// <summary>Typesafe wrapper for <c>Enum.ParseEnum()</c> that automatically checks 
  /// constants for membership in the <c>enum</c>.</summary>
  public static TEnum ParseEnum<TEnum>(string value, bool checkConstants) where TEnum : struct {
    TEnum enumValue;
    if (!TryParseEnum<TEnum>(value, out enumValue) && checkConstants) 
          throw new ArgumentOutOfRangeException("value",value,"Enum type: " + typeof(TEnum).Name);

    return enumValue;
  }

  /// <summary>Typesafe wrapper for <c>Enum.TryParseEnum()</c> that automatically checks 
  /// constants for membership in the <c>enum</c>.</summary>
  public static bool TryParseEnum<TEnum>(string value, out TEnum enumValue) where TEnum : struct {
    return Enum.TryParse<TEnum>(value, out enumValue)  
       &&  Enum.IsDefined(typeof(TEnum),enumValue);
  }

  /// <summary>Typesafe wrapper for <c>Enum.ToObject()</c>.</summary>
  /// <typeparam name="TEnum"></typeparam>
  public static TEnum EnumParse<TEnum>(char c, string lookup) {
    if (lookup==null) throw new ArgumentNullException("lookup");
    var index = lookup.IndexOf(c);
    if (index == -1) throw new ArgumentOutOfRangeException("c",c,"Enum Type: " + typeof(TEnum).Name);

    return (TEnum) Enum.ToObject(typeof(TEnum), index);
  }
  #endregion
}

在LINQ数据库查询中从Integer返回枚举名称

2 个答案: