使用C#调用Google Drive API时Files.Get触发NullReferenceException异常

我的项目需要实现以下功能：创建电子表格、填充内容、将表格移动到指定文件夹并锁定。目前创建和填充表格的功能正常，但执行移动文件操作时，在代码行var getRequest = DriveService.Files.Get(fileId);处抛出以下异常：

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.

相关代码如下：

static void Init()
{

    UserCredential credential;

    //Reading Credentials File...
    using (var stream = new FileStream("client_secret.json", FileMode.Open, FileAccess.Read))
    {
        string credPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);               
        credPath = Path.Combine(credPath, ".credentials/sheets.googleapis.com-dotnet-quickstart.json");

        credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
            GoogleClientSecrets.FromStream(stream).Secrets,
            Scopes,
            "user",
            CancellationToken.None,
            new FileDataStore(credPath, true)).Result;

    }

    // Creating Google Sheets API service...
    SheetsService = new SheetsService(new BaseClientService.Initializer()
    {
        HttpClientInitializer = credential,
        ApplicationName = ApplicationName,
    });

    // Create a Drive API service client
    DriveService DriveService = new DriveService(new Google.Apis.Services.BaseClientService.Initializer
    {
        HttpClientInitializer = credential,
        ApplicationName = ApplicationName,
    });

}

static void CreateSheet()
{

    // Create a new sheet with a single tab
    string sheetName = "Test Create Sheet";
    var NewSheet = new Google.Apis.Sheets.v4.Data.Spreadsheet();
    NewSheet.Properties = new SpreadsheetProperties();
    NewSheet.Properties.Title = sheetName;
    
    var newSheet = SheetsService.Spreadsheets.Create(NewSheet).Execute();

    SpreadsheetId = newSheet.SpreadsheetId;

    Sheet = "Sheet1";

}

static void MoveSheet()
{

    // Get the ID of the sheet you want to move
    string sheetId = SpreadsheetId;

    // Get the ID of the folder you want to move the sheet to
    string folderId = "XXXXXBeNF2rG0PA6bsi89YugahH-XXXX";

    // Retrieve the existing parents to remove
    string fileId = sheetId;

    var getRequest = DriveService.Files.Get(fileId);
    getRequest.Fields = "parents";
    var file = getRequest.Execute();
    var previousParents = String.Join(",", file.Parents);

    // Move the file to the new folder
    var updateRequest =
        DriveService.Files.Update(new Google.Apis.Drive.v3.Data.File(),
            fileId);
    updateRequest.Fields = "id, parents";
    updateRequest.AddParents = folderId;
    updateRequest.RemoveParents = previousParents;
    //file = updateRequest.Execute();

}

问题根源 #

你在Init方法中创建的DriveService是局部变量，而MoveSheet方法中引用的是类的静态DriveService成员（从代码中SheetsService的使用方式可推断），这导致MoveSheet里的DriveService未被初始化，始终为null，从而触发空引用异常。

修复步骤 #

1. 确保类级别声明了静态的DriveService成员，与SheetsService对应：

// 在类的顶部添加静态成员声明
private static SheetsService SheetsService;
private static DriveService DriveService;

2. 修改Init方法中的代码，将局部的DriveService赋值改为给静态成员赋值，去掉局部变量的声明：

// Create a Drive API service client
DriveService = new DriveService(new Google.Apis.Services.BaseClientService.Initializer
{
    HttpClientInitializer = credential,
    ApplicationName = ApplicationName,
});

完成以上修改后，MoveSheet方法就能正确引用到已初始化的DriveService实例，避免空引用异常。

内容的提问来源于stack exchange，提问作者warrenk