How to write and read multiple types of data using Data Writer and Data Reader

Question

How to write and read multiple types of data using Data Writer and Data Reader

fangfang wu 91 Microsoft Employee Moderator

I need to write a file length, file data and its type as a string value to the stream.

I’m using the following code with Data Writer.
writer = new DataWriter(socket.OutputStream);

// if we have a StorageFile named file  
using (var stream=await file.OpenStreamForReadAsync())  
using(var ms=new MemoryStream())  
{  
    await stream.CopyToAsync(ms);  
    var bytes = ms.ToArray();  
    writer.WriteUInt32(Convert.ToUInt32(bytes.Length));  
    writer.WriteString("Video");  
    writer.WriteBytes(ms.ToArray());  
    await writer.StoreAsync();  
}

And I tried to read that value using Data Reader as following.

try  
{  
    while (true)  
    {  
	//Read first 4 bytes(length of the subsequent string).  
	uint sizeFieldCount = await reader.LoadAsync(sizeof(uint));  
  	if (sizeFieldCount != sizeof(uint))  
        {  
           // The underlying socket was closed before we were able to read the whole data.  
            return;  
        }  
  
	// Read the string.  
	uint fileLength = reader.ReadUInt32();  
	uint actualFileLength = await reader.LoadAsync(fileLength);  
	if (fileLength != actualFileLength)  
    	{  
	    // The underlying socket was closed before we were able to read the whole data.  
	    return;  
	}  
	String type = reader.ReadString(5);  
  
 	var bytes = new byte[actualFileLength];  
	reader.ReadBytes(bytes);  
                  
	 var file = await ApplicationData.Current.LocalFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);  
         await FileIO.WriteBytesAsync(file, bytes);  
    }  
}  
catch (Exception exception)  
{  
       // If this is an unknown status it means that the error is fatal and retry will likely fail.  
  
}

But the problem is that when I’m reading the data using Data Reader, I can't read file data. It throws System Exception.
Is there a way to write and read multiple types of data using Data Writer and Data Reader?

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Answer 1

Hello,

Welcome to our Microsoft Q&A platform!

First, the reason why you have the error is that the DataReader is not fully loaded.
Your data consists of uint, string and byte array.

uint fileLength = reader.ReadUInt32();  
uint actualFileLength = await reader.LoadAsync(fileLength);

This above code indicates that you loaded the fileLength length data in the DataReader, but in fact, this part of the data contains a string value of length 5.
So when you read the string value, the remaining data is not enough to fill your byte array, as a result, you will get an error.
To meet your needs, you need to call LoadAsync method twice.
Like this:

uint sizeFieldCount = await reader.LoadAsync(sizeof(uint));  
if (sizeFieldCount != sizeof(uint))  
{   
    return;  
}  
uint fileLength = reader.ReadUInt32();  
// Read the string.  
await reader.LoadAsync(5);  
string type = reader.ReadString(5);  
// Read the bytes.  
uint actualFileLength = await reader.LoadAsync(fileLength);  
if (fileLength != actualFileLength)  
{  
    return;  
}  
var bytes = new byte[actualFileLength];  
reader.ReadBytes(bytes);

Thanks

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How to write and read multiple types of data using Data Writer and Data Reader

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