如何在C# MAUI .NET 7.0中从ImageSource获取Base64字符串

问题背景 #

在C# MAUI .NET 7.0环境中，使用SyncFusion Signature Pad组件的ToImageSource()方法获取到StreamImageSource对象，尝试将其转换为Base64字符串时遇到问题。最初尝试将ImageSource赋值给Image控件immagine的Source属性，再调用SaveAsImage()方法保存，但读取文件时抛出System.IO.FileNotFoundException异常。

原尝试代码 #

private void btnConfermaClicked(object sender, EventArgs e)
{
    Log.Verbose("Signature confirm clicked");

    ImageSource source_firma = signaturePad.ToImageSource();
    Log.Debug($"Signature as {source_firma.GetType()}");

    immagine.Source = source_firma;

    // Generate random file name
    string fileName = Path.GetRandomFileName() + ".png";

    string folderName = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
    string path = Path.Combine(folderName, fileName);
    Log.Debug($"Random path: {path}");

    Log.Verbose("Start saving image");
    try
    {
        immagine.SaveAsImage(path);
    } catch (Exception ex)
    {
        Log.Error($"Error while saving image: {ex.Message}");
    }
    Log.Verbose("Done saving image");

    byte[] bytes = File.ReadAllBytes(path);
    string base64 = Convert.ToBase64String(bytes);
    Log.Debug($"Signature base64: {base64}");
} 

日志输出 #

[0:] [19:15:49 VRB] Signature confirm clicked
[0:] [19:15:49 DBG] Signature as Microsoft.Maui.Controls.StreamImageSource
[0:] [19:15:49 DBG] Random path: /data/user/0/com.companyname.iglogestorepresenze/files/eaiktmuj.0gr.png
[0:] [19:15:49 VRB] Start saving image
[gestorepresenz] Explicit concurrent copying GC freed 12995(775KB) AllocSpace objects, 7(5008KB) LOS objects, 84% free, 4446KB/28MB, paused 32us total 14.729ms
[0:] [19:15:49 VRB] Done saving image
**System.IO.FileNotFoundException:** 'Could not find file '/data/user/0/com.companyname.iglogestorepresenze/files/eaiktmuj.0gr.png'.'

问题原因 #

1. SaveAsImage()是异步操作，但原代码同步调用后立即读取文件，此时文件可能还未完成写入，导致找不到文件；
2. 无需通过Image控件中转保存，直接从StreamImageSource获取流即可转换为Base64，步骤更简洁且避免异步问题。

解决方案 #

方案1：使用异步保存方法（修正原思路） #

将方法改为异步，调用SaveAsImageAsync()并等待完成后再读取文件：

private async void btnConfermaClicked(object sender, EventArgs e)
{
    Log.Verbose("Signature confirm clicked");

    ImageSource source_firma = signaturePad.ToImageSource();
    Log.Debug($"Signature as {source_firma.GetType()}");

    immagine.Source = source_firma;

    string fileName = Path.GetRandomFileName() + ".png";
    string folderName = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
    string path = Path.Combine(folderName, fileName);
    Log.Debug($"Random path: {path}");

    Log.Verbose("Start saving image");
    try
    {
        await immagine.SaveAsImageAsync(path);
    } catch (Exception ex)
    {
        Log.Error($"Error while saving image: {ex.Message}");
        return; // 保存失败直接返回，避免后续读取错误
    }
    Log.Verbose("Done saving image");

    if (File.Exists(path))
    {
        byte[] bytes = File.ReadAllBytes(path);
        string base64 = Convert.ToBase64String(bytes);
        Log.Debug($"Signature base64: {base64}");
        // 可选：删除临时文件
        File.Delete(path);
    }
    else
    {
        Log.Error("File still not found after saving");
    }
}

方案2：直接从StreamImageSource获取流转换Base64（更高效） #

跳过Image控件，直接从StreamImageSource获取流并转换为Base64，无需写入文件：

private async void btnConfermaClicked(object sender, EventArgs e)
{
    Log.Verbose("Signature confirm clicked");

    ImageSource source_firma = signaturePad.ToImageSource();
    Log.Debug($"Signature as {source_firma.GetType()}");

    if (source_firma is StreamImageSource streamSource)
    {
        using (var stream = await streamSource.Stream(CancellationToken.None))
        {
            using (var memoryStream = new MemoryStream())
            {
                await stream.CopyToAsync(memoryStream);
                byte[] bytes = memoryStream.ToArray();
                string base64 = Convert.ToBase64String(bytes);
                Log.Debug($"Signature base64: {base64}");
            }
        }
    }
    else
    {
        Log.Error("Unsupported ImageSource type");
    }
}

补充说明 #

immagine是XAML中定义的Image控件，正确代码应为：<Image x:Name="immagine" />（注：原代码中id应为MAUI标准的x:Name属性，用于标识控件）

内容的提问来源于stack exchange，提问作者TimoFran