在使用語句中操作流

Question

我正在處理.NET Core中的上傳，它們以IFormFile傳遞。在我最終傳遞要保存的文件之前，我需要操作流來移除Exif數據。但是，我不知道如何處理這個問題。我正在嘗試下面的代碼，但它不起作用，因爲stream只能在using聲明中使用。

什麼是正確的方式來做這樣的事情？

public async Task<IActionResult> UploadImage(IFormFile image) 
{ 
    using (var stream = new MemoryStream()) 
    { 
     // image is IFormFile 
     await image.CopyToAsync(stream); 

     // doesn't work, stream is readonly 
     stream = StripExif(stream); 

     // save stream 
    } 

    return Ok(); 
} 

public static Stream StripExif(Stream stream) 
{ 
    stream.Seek(0, SeekOrigin.Begin); 

    // remove Exif data 

    return stream; 
} 

Answer 1

StripExif實際上不是創建一個新的Stream，相反，它的變異的Stream傳遞給它，所以它不應該在所有將返回一個Stream。當呼叫者然後修改呼叫以反映其變異的參數，而不是創建一個新的流，問題消失：

public async Task<IActionResult> UploadImage(IFormFile image) 
{ 
    using (var stream = new MemoryStream()) 
    { 
     // image is IFormFile 
     await image.CopyToAsync(stream); 

     StripExif(stream); 

     // save stream 
    } 

    return Ok(); 
} 

public static void StripExif(Stream stream) 
{ 
    stream.Seek(0, SeekOrigin.Begin); 

    // remove Exif data 
} 

Answer 2

使用只是有一個問題，因爲你試圖分配給使用變量。您可以在StripExif中創建新的MemoryStream，然後將StripExif的結果分配給新的流變量。

public async Task<IActionResult> UploadImage(IFormFile image) 
{ 
    using (var stream = new MemoryStream()) 
    { 
     // image is IFormFile 
     await image.CopyToAsync(stream); 

     var strippedStream = StripExif(stream); 

     // save strippedStream, not stream. 
    } 

    return Ok(); 
} 

public static MemoryStream StripExif(MemoryStream stream) 
{ 
    var result = new MemoryStream(); 

    stream.Seek(0, SeekOrigin.Begin); 

    // remove Exif data from result 

    return result; 
} 

更新時間：

public async Task<IActionResult> UploadImage(IFormFile image) 
{ 
    using (var stream = new MemoryStream()) 
    using (var writer = new StreamWriter(new MemoryStream()) 
    { 
     // image is IFormFile 
     await image.CopyToAsync(stream); 

     var strippedStream = StripExif(stream, writer); 

     // save strippedStream, not stream. 
    } 

    return Ok(); 
} 

public static void StripExif(MemoryStream stream, StreamWriter writer) 
{ 
    var result = new MemoryStream(); 

    // lets assume for sake of example that removing ExIf just removes the 
    // the first 10 characters of the stream 
    stream.Seek(10, SeekOrigin.Begin); 

    writer.Write(stream.ReadToEnd());   
}