c# - 检查什么类型的枚举进入了通用方法

Question

我现在想检查传入的列以查看它是什么类型的枚举，并基于此返回该列的正确数据库类型。我无法让它工作，不知道如何:

  public DbType GetColumnDbType<T>(T column) where T: IComparable, IFormattable, IConvertible
        {
            if (!typeof(T).IsEnum) throw new ArgumentException("the object passed in must an enum type");


            switch (column)
            {
                case (Enums.MemberColumn)column: 
                    switch (Enums.MemberColumn)
                    {
                        case Enums.MemberColumn.Address1:
                            return DbType.String;
                            break;
                        case Enums.MemberColumn.City:
                            return DbType.String;
                            break;
                        case Enums.MemberColumn.State:
                            return DbType.String;
                            break;
                    }
            }
   .....

更新

这是我尝试过的，但编译器在第一行提示说“期望整数类型的值”

    switch(typeof(T))
    {
        case (typeof (Enums.MemberColumn)):
            switch ((Enums.MemberColumn) column)
            {
                case Enums.MemberColumn.Address1:
                    return DbType.String;
                    break;
                case Enums.MemberColumn.City:
                    return DbType.String;
                    break;
                case Enums.MemberColumn.State:
                    return DbType.String;
                    break;
                default:
                    throw new ArgumentException("Unsupported enum type.", "MemberColumn");
                    break;
            }
    }

Answer 1

我觉得你的 switch 语句有点可笑。试试这个:

        if (typeof(T) == typeof(Enums.MemberColumn))
        {
            switch ((Enums.MemberColumn)column)
            {
                case Enums.MemberColumn.Address1:

         ....

正如其他人所建议的那样，如果您要根据其类型进行切换，那么使用泛型方法有什么意义呢？只需这样做:

public DbType GetColumnDbType(Enums.MemberColumn column)
{
     switch(column)
     {
         case (Enums.MemberColumn.Address1):

         ....

}

然后为其他枚举类型创建另一个重载方法:

public DbType GetColumnDbType(Enums.OtherMemberColumn column)
{
     switch(column)
     {
          .....
     }

}

或者...我会这样做的方式:

[AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
public class DbTypeAttribute : Attribute
{
    public DbType DbType { get; private set; }

    public DbTypeAttribute(DbType dbType)
    {
        this.DbType = dbType;
    }
}

public static class DbHelper<T> where T : IComparable, IFormattable, IConvertible, struct
{
    private static Dictionary<long, DbType> _dbTypeLookup = CreateLookup();

    public static DbType GetColumnType(T column)
    {
        return _dbTypeLookup[(long)(object)column];
    }

    private static Dictionary<long, DbType> CreateLookup()
    {   
        if (!typeof(T).IsEnum)
            throw new InvalidOperationException("T must be an enum type.");

        var dbTypeLookup = new Dictionary<long, DbType>();

        foreach (var name in Enum.GetNames(typeof(T)))
        {
            var enumMember = typeof(T).GetMember(name).Single();
            var dbTypeAttr = (DbTypeAttribute)enumMember.GetCustomAttributes(typeof(DbTypeAttribute), false).Single();

            dbTypeLookup.Add((long)Enum.Parse(typeof(T), name), dbTypeAttr.DbType);
        }

        return dbTypeLookup;
    }
}

有了上面的代码，我现在可以这样做了:

public enum MemberColumn
{
    [DbType(DbType.String)] Address1,
    [DbType(DbType.String)] Address2,
    [DbType(DbType.String)] FirstName,
    [DbType(DbType.String)] LastName,
    [DbType(DbType.DateTime)] DateOfBirth,
}


// ... later, in some method somewhere:

MemberColumn c = MemberColumn.Address1;

DbType dbType = DbHelper<MemberColumn>.GetColumnType(c); // sets dbType to DbType.String