实例化泛型类型时出现InvalidCastException

Question

下面的代码给了我一个InvalidCastException ，声明我不能在foreach循环中从源类型InvalidCastException为目标类型。 我已经尝试通过该方法传递多个不同的泛型集合，我总是得到这个错误。 我无法弄清楚为什么。 任何帮助，将不胜感激。

public static void WriteDataListToFile<T>(T dataList, string folderPath, string fileName) where T : IEnumerable, ICollection
{    
     //Check to see if file already exists
     if(!File.Exists(folderPath + fileName))
     {
         //if not, create it
         File.Create(folderPath + fileName);
     }

     using(StreamWriter sw = new StreamWriter(folderPath + fileName))
     {
         foreach(T type in dataList)
         {
             sw.WriteLine(type.ToString());
         }
     }
}

Answer 1

您的dataList应该是IEnumerable<T>

public static void WriteDataListToFile<T>(IEnumerable<T> dataList, string folderPath, string fileName)
{
    //Check to see if file already exists
    if (!File.Exists(folderPath + fileName))
    {
        //if not, create it
        File.Create(folderPath + fileName);
    }

    using (StreamWriter sw = new StreamWriter(folderPath + fileName))
    {
        foreach (T type in dataList)
        {
            sw.WriteLine(type.ToString());
        }
    }
}

Answer 2

像这样使用var ：

foreach (var type in dataList)
{
    sw.WriteLine(type.ToString());
}

Answer 3

您试图将列表中的每个项目键入为T ，但您的类型约束会强制T为IEnumerable 。 我想你想将你的参数指定为IEnumerable<T>并删除类型约束：

public static void WriteDataListToFile<T>(IEnumerable<T> dataList, string folderPath, string fileName) //no type constraints
{
    //your other things
    foreach(T type in dataList)
    {
        sw.WriteLine(type.ToString());
    }
}

Answer 4

您应该尝试使用Cast<T>在foreach构建集合。 像这样：

public static void WriteDataListToFile<T>(T dataList, string folderPath, string fileName) where T : IEnumerable, ICollection
 {    
     //Check to see if file already exists
     if(!File.Exists(folderPath + fileName))
     {
         //if not, create it
         File.Create(folderPath + fileName);
     }

     using(StreamWriter sw = new StreamWriter(folderPath + fileName))   
     {
        // added Cast<T>
         foreach(T type in dataList.Cast<T>())
         {
             sw.WriteLine(type.ToString());
         }
     }
}