1. 在JavaScript中编写保存canvas内容为DataURL的函数：

function saveCanvasToDataURL(canvasId) {
  var canvas = document.getElementById(canvasId);
  return canvas.toDataURL();
}

2. 在Blazor组件中使用JS Interop调用JavaScript函数，并将返回的DataURL作为参数传递给C#方法：

using Microsoft.JSInterop;

public class CanvasComponent : ComponentBase
{
  [Inject]
  private IJSRuntime JSRuntime { get; set; }

  private string imageDataUrl = "";

  protected override async Task OnAfterRenderAsync()
  {
    var canvasId = "myCanvas";

    // call JS function to save canvas to data URL
    imageDataUrl = await JSRuntime.InvokeAsync("saveCanvasToDataURL", canvasId);

    StateHasChanged();
  }

  private void SaveImageDataUrl()
  {
    Console.WriteLine(imageDataUrl);
    // do something with the image data URL
  }
}