CODEWITHSUNDEEP
c#exceptiongenericscasting
Matthew Sawrey
Matthew Sawrey

Reputation: 105

InvalidCastException when instancing generic type

The following code gives me an InvalidCastException, stating that I can't cast from source to destination type in the foreach loop. I've tried passing multiple different generic collections through the method and I always get this error. I can't figure out why. Any help would be appreciated.

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());
         }
     }
}

Upvotes: 3

Views: 69

Answers (4)

Jakub Lortz
Jakub Lortz

Reputation: 14896

You dataList should be an 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());
        }
    }
}

Upvotes: 4

Salah Akbari
Salah Akbari

Reputation: 39946

Use var like this:

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

Upvotes: 2

Dave Zych
Dave Zych

Reputation: 21887

You're attempting to type each item inside the list as T, but your type constraints force T to be IEnumerable. I think you want to specify your parameter as IEnumerable<T> and to remove the type constraints:

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());
    }
}

Upvotes: 1

Glenn Ferrie
Glenn Ferrie

Reputation: 10390

You should try casting your collection within the foreach using Cast<T>. like this:

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());
         }
     }
}

Upvotes: 0

Related Questions