在写入文件之前创建文件并分配存储

Question

I have a file that gets GBs of data written to it over time (looping once it reaches the end). I would like to create the file ahead of time and preset the storage so that the required storage is never taken up by other downloads during the writing to the file. This is done using visual studio 2012 in C#.

I have tried:

 if (fileSizeRequirement or fileName is changed) //if filePath or file size is changed 
        {   
            //Open or create the file, set the file to size requirement, close the filestream               
            fileStream = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite);           
            fileStream.SetLength((long)fileSizeRequirement);
            fileStream.Close();
        }

1) Is this an appropriate way to "preallocate" space for a folder? 2) Will the SetLength require a seek to the beginning after setting the length or does the position in the folder stay at the beginning? 3) What is the correct way to achieve file preallocation of storage space?

Thanks ahead of time and I appreciate any suggestions.

Answer 1

Using SetLength is a common approach although I'd generally use a using statement here.

using(var fileStream = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
    fileStream.SetLength((long)fileSizeRequirement);
}

Calling fileStream.Position straight after SetLength yields 0 so you shouldn't need to seek to the beginning.