创建、写入和读取文件

重要的 API

• StorageFolder 类
• StorageFile 类
• FileIO 类

使用 StorageFile 对象读取和写入文件。

注意

 有关完整示例，请参阅文件访问示例。

必备条件

• 了解通用 Windows 平台 (UWP) 应用的异步编程

  若要了解如何使用 C# 或 Visual Basic 编写异步应用，请参阅使用 C# 或 Visual Basic 调用异步 API。 若要了解如何在 C++/WinRT 中编写异步应用，请参阅借助 C++/WinRT 的并发和异步操作。 若要了解如何使用 C++/CX 编写异步应用，请参阅使用 C++/CX 进行异步编程。

• 了解如何获取你希望读取和/或写入的文件

  可以在使用选取器打开文件和文件夹中了解如何使用文件选取器获取文件。

创建文件

这里介绍了如何在应用的本地文件夹中创建一个文件。 如果文件存在，我们将替换它。

// Create sample file; replace if exists.
Windows.Storage.StorageFolder storageFolder =
    Windows.Storage.ApplicationData.Current.LocalFolder;
Windows.Storage.StorageFile sampleFile =
    await storageFolder.CreateFileAsync("sample.txt",
        Windows.Storage.CreationCollisionOption.ReplaceExisting);

// MainPage.h
#include <winrt/Windows.Storage.h>
...
Windows::Foundation::IAsyncAction ExampleCoroutineAsync()
{
    // Create a sample file; replace if exists.
    Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
    co_await storageFolder.CreateFileAsync(L"sample.txt", Windows::Storage::CreationCollisionOption::ReplaceExisting);
}

// Create a sample file; replace if exists.
StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
concurrency::create_task(storageFolder->CreateFileAsync("sample.txt", CreationCollisionOption::ReplaceExisting));

' Create sample file; replace if exists.
Dim storageFolder As StorageFolder = Windows.Storage.ApplicationData.Current.LocalFolder
Dim sampleFile As StorageFile = Await storageFolder.CreateFileAsync("sample.txt", CreationCollisionOption.ReplaceExisting)

写入文件

下面介绍了如何使用 StorageFile 类在磁盘上写入可写文件。 每种写入文件（除非你在创建后立即写入文件）的方法的常见第一步是使用 StorageFolder.GetFileAsync 获取文件。

Windows.Storage.StorageFolder storageFolder =
    Windows.Storage.ApplicationData.Current.LocalFolder;
Windows.Storage.StorageFile sampleFile =
    await storageFolder.GetFileAsync("sample.txt");

// MainPage.h
#include <winrt/Windows.Storage.h>
...
Windows::Foundation::IAsyncAction ExampleCoroutineAsync()
{
    Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
    auto sampleFile{ co_await storageFolder.CreateFileAsync(L"sample.txt", Windows::Storage::CreationCollisionOption::ReplaceExisting) };
    // Process sampleFile
}

StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile) 
{
    // Process file
});

Dim storageFolder As StorageFolder = Windows.Storage.ApplicationData.Current.LocalFolder
Dim sampleFile As StorageFile = Await storageFolder.GetFileAsync("sample.txt")

将文本写入文件

通过调用 FileIO.WriteTextAsync 方法，将文本写入文件 。

await Windows.Storage.FileIO.WriteTextAsync(sampleFile, "Swift as a shadow");

// MainPage.h
#include <winrt/Windows.Storage.h>
...
Windows::Foundation::IAsyncAction ExampleCoroutineAsync()
{
    Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
    auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
    // Write text to the file.
    co_await Windows::Storage::FileIO::WriteTextAsync(sampleFile, L"Swift as a shadow");
}

StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile) 
{
    //Write text to a file
    create_task(FileIO::WriteTextAsync(sampleFile, "Swift as a shadow"));
});

Await Windows.Storage.FileIO.WriteTextAsync(sampleFile, "Swift as a shadow")

使用缓冲区将字节写入文件（2 步）

1. 首先，调用 CryptographicBuffer.ConvertStringToBinary 以获取你想要写入文件的字节（基于字符串）的缓冲区 。

   var buffer = Windows.Security.Cryptography.CryptographicBuffer.ConvertStringToBinary(
       "What fools these mortals be", Windows.Security.Cryptography.BinaryStringEncoding.Utf8);
   

   // MainPage.h
   #include <winrt/Windows.Security.Cryptography.h>
   #include <winrt/Windows.Storage.h>
   #include <winrt/Windows.Storage.Streams.h>
   ...
   Windows::Foundation::IAsyncAction ExampleCoroutineAsync()
   {
       Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
       auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
       // Create the buffer.
       Windows::Storage::Streams::IBuffer buffer{
           Windows::Security::Cryptography::CryptographicBuffer::ConvertStringToBinary(
               L"What fools these mortals be", Windows::Security::Cryptography::BinaryStringEncoding::Utf8)};
       // The code in step 2 goes here.
   }
   

   StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
   create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
   {
       // Create the buffer
       IBuffer^ buffer = CryptographicBuffer::ConvertStringToBinary
       ("What fools these mortals be", BinaryStringEncoding::Utf8);
   });
   

   Dim buffer = Windows.Security.Cryptography.CryptographicBuffer.ConvertStringToBinary(
       "What fools these mortals be",
       Windows.Security.Cryptography.BinaryStringEncoding.Utf8)
   

2. 然后，通过调用 FileIO.WriteBufferAsync 方法，将字节从缓冲区写入文件 。

   await Windows.Storage.FileIO.WriteBufferAsync(sampleFile, buffer);
   

   co_await Windows::Storage::FileIO::WriteBufferAsync(sampleFile, buffer);
   

   StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
   create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
   {
       // Create the buffer
       IBuffer^ buffer = CryptographicBuffer::ConvertStringToBinary
       ("What fools these mortals be", BinaryStringEncoding::Utf8);      
       // Write bytes to a file using a buffer
       create_task(FileIO::WriteBufferAsync(sampleFile, buffer));
   });
   

   Await Windows.Storage.FileIO.WriteBufferAsync(sampleFile, buffer)
   

使用流将文本写入文件（4 步）

1. 首先，调用 StorageFile.OpenAsync 方法打开文件。 打开操作完成后，它将返回文件的内容流。

   var stream = await sampleFile.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite);
   

   // MainPage.h
   #include <winrt/Windows.Storage.h>
   #include <winrt/Windows.Storage.Streams.h>
   ...
   Windows::Foundation::IAsyncAction ExampleCoroutineAsync()
   {
       Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
       auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
       Windows::Storage::Streams::IRandomAccessStream stream{ co_await sampleFile.OpenAsync(Windows::Storage::FileAccessMode::ReadWrite) };
       // The code in step 2 goes here.
   }
   

   StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
   create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
   {
       create_task(sampleFile->OpenAsync(FileAccessMode::ReadWrite)).then([sampleFile](IRandomAccessStream^ stream)
       {
           // Process stream
       });
   });
   

   Dim stream = Await sampleFile.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite)
   

2. 接下来，通过从 stream 调用 IRandomAccessStream.GetOutputStreamAt 方法获取输出流 。 如果你使用的是 C#，则将其附在 using 语句中以管理输出流的生存期 。 如果你使用的是 C++/WinRT，则可以通过将其附在块中，或者通过在使用结束时将其设置为 nullptr 来控制其生存期。

   using (var outputStream = stream.GetOutputStreamAt(0))
   {
       // We'll add more code here in the next step.
   }
   stream.Dispose(); // Or use the stream variable (see previous code snippet) with a using statement as well.
   

   Windows::Storage::Streams::IOutputStream outputStream{ stream.GetOutputStreamAt(0) };
   // The code in step 3 goes here.
   

   // Add to "Process stream" in part 1
   IOutputStream^ outputStream = stream->GetOutputStreamAt(0);
   

   Using outputStream = stream.GetOutputStreamAt(0)
   ' We'll add more code here in the next step.
   End Using
   

3. 现在，添加此代码（如果你使用的是 C#，在现有 using 语句中），以通过创建一个新的 DataWriter 对象和调用 DataWriter.WriteString 方法写入输出流 。

   using (var dataWriter = new Windows.Storage.Streams.DataWriter(outputStream))
   {
       dataWriter.WriteString("DataWriter has methods to write to various types, such as DataTimeOffset.");
   }
   

   Windows::Storage::Streams::DataWriter dataWriter;
   dataWriter.WriteString(L"DataWriter has methods to write to various types, such as DataTimeOffset.");
   // The code in step 4 goes here.
   

   // Added after code from part 2
   DataWriter^ dataWriter = ref new DataWriter(outputStream);
   dataWriter->WriteString("DataWriter has methods to write to various types, such as DataTimeOffset.");
   

   Dim dataWriter As New DataWriter(outputStream)
   dataWriter.WriteString("DataWriter has methods to write to various types, such as DataTimeOffset.")
   

4. 最后，添加此代码（如果你使用的是 C#，在内部 using 语句中）以使用 DataWriter.StoreAsync 将文本保存到你的文件并使用 IOutputStream.FlushAsync 关闭该流 。

   await dataWriter.StoreAsync();
   await outputStream.FlushAsync();
   

   dataWriter.StoreAsync();
   outputStream.FlushAsync();
   

   // Added after code from part 3
   dataWriter->StoreAsync();
   outputStream->FlushAsync();
   

   Await dataWriter.StoreAsync()
   Await outputStream.FlushAsync()
   

向文件进行写入的最佳做法

有关其他详细信息和最佳做法指南，请参阅。

从文件读取

下面介绍了如何使用 StorageFile 类在磁盘上从文件进行读取。 每种从文件读取的方法的常见第一步是使用 StorageFolder.GetFileAsync 获取文件。

Windows.Storage.StorageFolder storageFolder =
    Windows.Storage.ApplicationData.Current.LocalFolder;
Windows.Storage.StorageFile sampleFile =
    await storageFolder.GetFileAsync("sample.txt");

Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
// Process file

StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
{
    // Process file
});

Dim storageFolder As StorageFolder = Windows.Storage.ApplicationData.Current.LocalFolder
Dim sampleFile As StorageFile = Await storageFolder.GetFileAsync("sample.txt")

从文件读取文本

通过调用 FileIO.ReadTextAsync 方法，从文件读取文本 。

string text = await Windows.Storage.FileIO.ReadTextAsync(sampleFile);

Windows::Foundation::IAsyncOperation<winrt::hstring> ExampleCoroutineAsync()
{
    Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
    auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
    co_return co_await Windows::Storage::FileIO::ReadTextAsync(sampleFile);
}

StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
{
    return FileIO::ReadTextAsync(sampleFile);
});

Dim text As String = Await Windows.Storage.FileIO.ReadTextAsync(sampleFile)

通过使用缓冲区从文件读取文本（2 步）

1. 首先，调用 FileIO.ReadBufferAsync 方法。

   var buffer = await Windows.Storage.FileIO.ReadBufferAsync(sampleFile);
   

   Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
   auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
   Windows::Storage::Streams::IBuffer buffer{ co_await Windows::Storage::FileIO::ReadBufferAsync(sampleFile) };
   // The code in step 2 goes here.
   

   StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
   create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
   {
       return FileIO::ReadBufferAsync(sampleFile);
   
   }).then([](Streams::IBuffer^ buffer)
   {
       // Process buffer
   });
   

   Dim buffer = Await Windows.Storage.FileIO.ReadBufferAsync(sampleFile)
   

2. 然后，使用 DataReader 对象先读取缓冲区的长度，然后读取其内容。

   using (var dataReader = Windows.Storage.Streams.DataReader.FromBuffer(buffer))
   {
       string text = dataReader.ReadString(buffer.Length);
   }
   

   auto dataReader{ Windows::Storage::Streams::DataReader::FromBuffer(buffer) };
   winrt::hstring bufferText{ dataReader.ReadString(buffer.Length()) };
   

   // Add to "Process buffer" section from part 1
   auto dataReader = DataReader::FromBuffer(buffer);
   String^ bufferText = dataReader->ReadString(buffer->Length);
   

   Dim dataReader As DataReader = Windows.Storage.Streams.DataReader.FromBuffer(buffer)
   Dim text As String = dataReader.ReadString(buffer.Length)
   

使用流从文件读取文本（4 步）

1. 通过调用 StorageFile.OpenAsync 方法，为你的文件打开流。 操作完成后，它将返回文件的内容流。

   var stream = await sampleFile.OpenAsync(Windows.Storage.FileAccessMode.Read);
   

   Windows::Storage::StorageFolder storageFolder{ Windows::Storage::ApplicationData::Current().LocalFolder() };
   auto sampleFile{ co_await storageFolder.GetFileAsync(L"sample.txt") };
   Windows::Storage::Streams::IRandomAccessStream stream{ co_await sampleFile.OpenAsync(Windows::Storage::FileAccessMode::Read) };
   // The code in step 2 goes here.
   

   StorageFolder^ storageFolder = ApplicationData::Current->LocalFolder;
   create_task(storageFolder->GetFileAsync("sample.txt")).then([](StorageFile^ sampleFile)
   {
       create_task(sampleFile->OpenAsync(FileAccessMode::Read)).then([sampleFile](IRandomAccessStream^ stream)
       {
           // Process stream
       });
   });
   

   Dim stream = Await sampleFile.OpenAsync(Windows.Storage.FileAccessMode.Read)
   

2. 获取稍后要使用的流的大小。

   ulong size = stream.Size;
   

   uint64_t size{ stream.Size() };
   // The code in step 3 goes here.
   

   // Add to "Process stream" from part 1
   UINT64 size = stream->Size;
   

   Dim size = stream.Size
   

3. 通过调用 IRandomAccessStream.GetInputStreamAt 方法获取输入流 。 将其放到 using 语句中以管理流的生存期。 调用 GetInputStreamAt 时指定 0，以将位置设置在流的开头。

   using (var inputStream = stream.GetInputStreamAt(0))
   {
       // We'll add more code here in the next step.
   }
   

   Windows::Storage::Streams::IInputStream inputStream{ stream.GetInputStreamAt(0) };
   Windows::Storage::Streams::DataReader dataReader{ inputStream };
   // The code in step 4 goes here.
   

   // Add after code from part 2
   IInputStream^ inputStream = stream->GetInputStreamAt(0);
   auto dataReader = ref new DataReader(inputStream);
   

   Using inputStream = stream.GetInputStreamAt(0)
   ' We'll add more code here in the next step.
   End Using
   

4. 最后，将此代码添加到现有的 using 语句中以在流上获取 DataReader 对象，然后通过调用 DataReader.LoadAsync 和 DataReader.ReadString 读取文本。

   using (var dataReader = new Windows.Storage.Streams.DataReader(inputStream))
   {
       uint numBytesLoaded = await dataReader.LoadAsync((uint)size);
       string text = dataReader.ReadString(numBytesLoaded);
   }
   

   unsigned int cBytesLoaded{ co_await dataReader.LoadAsync(size) };
   winrt::hstring streamText{ dataReader.ReadString(cBytesLoaded) };
   

   // Add after code from part 3
   create_task(dataReader->LoadAsync(size)).then([sampleFile, dataReader](unsigned int numBytesLoaded)
   {
       String^ streamText = dataReader->ReadString(numBytesLoaded);
   });
   

   Dim dataReader As New DataReader(inputStream)
   Dim numBytesLoaded As UInteger = Await dataReader.LoadAsync(CUInt(size))
   Dim text As String = dataReader.ReadString(numBytesLoaded)
   

另请参阅

• 向文件进行写入的最佳做法