Prevent lost updates (ETag)

The following example shows you how to replace a file only if nobody changed it since you read it, and how to create a file only if it does not exist yet. When the condition does not hold the server answers 412 (Precondition Failed) and nothing is changed.

C# example

using System;
using System.IO;
using System.Net;
using System.Threading.Tasks;
using Independentsoft.Webdav;

namespace Sample
{
    class Program
    {
        static async Task Main(string[] args)
        {
            NetworkCredential credential = new NetworkCredential("username", "password");

            using (WebdavClient client = new WebdavClient(credential))
            {
                client.BaseAddress = new Uri("https://myserver/dav/");

                ResourceInfo file = await client.GetResource("documents/notes.txt");

                // ... edit the document ...

                try
                {
                    using (MemoryStream content = new MemoryStream(System.Text.Encoding.UTF8.GetBytes("Edited notes")))
                    {
                        UploadOptions options = new UploadOptions();
                        options.Precondition = Precondition.IfMatch(file.ETag);

                        await client.Upload("documents/notes.txt", content, options);
                        Console.WriteLine("Saved.");
                    }
                }
                catch (WebdavException e) when (e.StatusCode == HttpStatusCode.PreconditionFailed)
                {
                    Console.WriteLine("Someone else changed the file in the meantime.");
                }

                // Create a file, but never replace an existing one.
                try
                {
                    using (MemoryStream content = new MemoryStream(System.Text.Encoding.UTF8.GetBytes("New")))
                    {
                        UploadOptions options = new UploadOptions();
                        options.Precondition = Precondition.IfNotExists;

                        await client.Upload("documents/new.txt", content, options);
                        Console.WriteLine("Created documents/new.txt");
                    }
                }
                catch (WebdavException e) when (e.StatusCode == HttpStatusCode.PreconditionFailed)
                {
                    Console.WriteLine("documents/new.txt already exists.");
                }
            }
        }
    }
}

VB example

Imports System
Imports System.IO
Imports System.Net
Imports System.Threading.Tasks
Imports Independentsoft.Webdav

Namespace Sample
    Module Program
        Sub Main(args As String())
            RunAsync().GetAwaiter().GetResult()
        End Sub

        Private Async Function RunAsync() As Task
            Dim credential As New NetworkCredential("username", "password")

            Using client As New WebdavClient(credential)
                client.BaseAddress = New Uri("https://myserver/dav/")

                Dim info As ResourceInfo = Await client.GetResource("documents/notes.txt")

                ' ... edit the document ...

                Try
                    Using content As New MemoryStream(System.Text.Encoding.UTF8.GetBytes("Edited notes"))
                        Dim options As New UploadOptions()
                        options.Precondition = Precondition.IfMatch(info.ETag)

                        Await client.Upload("documents/notes.txt", content, options)
                        Console.WriteLine("Saved.")
                    End Using
                Catch e As WebdavException When e.StatusCode = HttpStatusCode.PreconditionFailed
                    Console.WriteLine("Someone else changed the file in the meantime.")
                End Try

                ' Create a file, but never replace an existing one.
                Try
                    Using content As New MemoryStream(System.Text.Encoding.UTF8.GetBytes("New"))
                        Dim options As New UploadOptions()
                        options.Precondition = Precondition.IfNotExists

                        Await client.Upload("documents/new.txt", content, options)
                        Console.WriteLine("Created documents/new.txt")
                    End Using
                Catch e As WebdavException When e.StatusCode = HttpStatusCode.PreconditionFailed
                    Console.WriteLine("documents/new.txt already exists.")
                End Try
            End Using
        End Function
    End Module
End Namespace

WebDAV .NET > Tutorial > Prevent lost updates (ETag)

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