Question

I am writing a Metro App.
I am trying to read a file and return a float[] from the data. But no matter what I do, the function seems to return null. I have tried the solutions to similar questions to no luck.

For example if I use:
float[] floatArray = new ModelReader(“filename.txt”).ReadModel()
The result will be a null array.

However if I use:
new ModelReader(“filename.txt”)
The correct array will be printed to the console because “Test” also prints the array before returning it. This seems very weird to me.

Please give me some guidance, I have no idea what is wrong.

public class ModelReader
{
    float[] array;

    public ModelReader(String name)
    {
        ReadModelAsync(name);
    }

    public float[] ReadModel()
    {
        return array;
    }

    private async Task ReadModelAsync(String name)
    {
        await readFile(name);
    }

    async Task readFile(String name)
    {
        // settings
        var path = @"Assets\models\" + name;
        var folder = Windows.ApplicationModel.Package.Current.InstalledLocation;

        // acquire file
        var file = await folder.GetFileAsync(path);

        // read content
        var read = await Windows.Storage.FileIO.ReadTextAsync(file);
        using (StringReader sr = new StringReader(read))
        {
            Test test = new Test(getFloatArray(sr));
            this.array = test.printArray();
        }
    }

    private float[] getFloatArray(StringReader sr) { ... }


public class Test
{
    public float[] floatArray;

    public Test(float[] floatArray) 
    {
        this.floatArray = floatArray;
    }


    public float[] printArray()
    {
        for (int i = 0; i < floatArray.Length; i++)
        {
            Debug.WriteLine(floatArray[i]);
        }

        return floatArray;
    }

}

Answer 1

You’re trying to get the result of an asynchronous operation before it has completed. I recommend you read my intro to async / await and follow-up with the async / await FAQ.

In particular, your constructor:

public ModelReader(String name)
{
  ReadModelAsync(name);
}

is returning before ReadModelAsync is complete. Since constructors cannot be asynchronous, I recommend you use an asynchronous factory or asynchronous lazy initialization as described on my blog (also available in my AsyncEx library).

Here’s a simple example using an asynchronous factory approach:

public class ModelReader
{
  float[] array;

  private ModelReader()
  {
  }

  public static async Task<ModelReader> Create(string name)
  {
    var ret = new ModelReader();
    await ret.ReadModelAsync(name);
    return ret;
  }

  ...
}