JavaScript Integration for Resolving the "No Call Dispatcher Has Been Set" Error in ASP.NET Core Blazor Server

Handling JavaScript and .NET Integration in Blazor Server Applications

Integrating JavaScript with.NET functions might occasionally result in unexpected issues when using Blazor Server. The "No call dispatcher has been set" error that appears when calling.NET functions from JavaScript is a frequent problem for developers. When attempting to call static.NET functions from outside of Blazor components, this problem can be quite annoying.

We will examine a typical situation where this problem arises in a Blazor Server application in this article. The problem usually shows up when you try to call a.NET method in JavaScript using the `window.DotNet` object, especially if the method is contained in a service rather than a component. For persistent tasks like logging, this method may be helpful.

We'll go over a real-world example that shows you how to configure your Blazor Server application to run a static auxiliary service. The intention is to guarantee that this service can communicate with JavaScript correctly, evading the typical mistakes that result in dispatcher errors. You'll see how these problems might be brought on by using the wrong namespace or by initializing the service incorrectly.

Lastly, we'll go over the actions required to fix the issue and guarantee that JavaScript may call your.NET methods with consistency. You can strengthen your Blazor Server application and make it more capable of supporting JavaScript interop by fixing these problems.

Understanding JavaScript to .NET Interoperability in Blazor Server

The problem of invoking a.NET method from JavaScript in a Blazor Server application is resolved by the given scripts. The main issue arises when programmers try to use JavaScript to call.NET functions but get the error "No call dispatcher has been set". This occurs because JavaScript cannot communicate with the.NET backend until the Blazor Server framework verifies that the call dispatcher is properly configured. In this example,.NET methods are stored in a static service class called JsHelperService, which makes them available worldwide and not restricted to a particular component that might decompose.

The [JSInvokable] core command is essential for making the .NET method callable from JavaScript. This attribute in the script designates the method WriteInfo, making it JavaScript-accessible. With the help of this message-logging service, you can see how to leverage.NET for centralized logging while keeping JavaScript open. The Init method should be used to call the service from Program.cs so that it is instantiated when the application starts and not dependent on separate components that may be disposed of.

The JavaScript portion of the example calls the.NET function asynchronously using window.DotNet.invokeMethodAsync. This makes sure that each method that is called is processed in a non-blocking manner, enabling other code to run in the interim while it waits for a.NET answer. The script creates a reusable method named writeInfo that can be invoked from any area of the program to log information by assigning this to the window.dotnetLogger object. For debugging, the script also uses a debugger line, which enables the developer to pause runtime and examine the status of variables.

Making ensuring the DotNet object is accessible in the global window scope is essential while troubleshooting. JavaScript is unable to invoke the.NET methods if this object is absent or configured incorrectly. The method namespace needs to be properly addressed in the invokeMethodAsync call in order to prevent the problem. Mismatching the namespace or failing to correctly register the service in Program.cs are common errors. The problem of service disposal is resolved by registering the service as a singleton using builder.Services.AddSingleton, which guarantees that the service is available for the duration of the application.

namespace MyNamespace.Utility
{
    public static class JsHelperService
    {
        static JsHelperService()
        {
            var i = 0; // Constructor breakpoint test
        }
        public static void Init() { /* Ensure initialization in Program.cs */ }
        [JSInvokable("WriteInfo")]
        public static void WriteInfo(string message)
        {
            Logger.Instance.WriteInfo(message);
        }
    }
}

namespace MyNamespace.Utility
{
    public class JsHelperService
    {
        private readonly ILogger _logger;
        public JsHelperService(ILogger<JsHelperService> logger)
        {
            _logger = logger;
        }
        [JSInvokable("WriteInfo")]
        public void WriteInfo(string message)
        {
            _logger.LogInformation(message);
        }
    }
}

// In Program.cs, register the service
builder.Services.AddSingleton<JsHelperService>();

function setupLogging() {
    debugger; // For debugging
    window.dotnetLogger = window.dotnetLogger || {};
    window.dotnetLogger.writeInfo = function (message) {
        window.DotNet.invokeMethodAsync('MyNamespace', 'WriteInfo', message)
        .then(response => console.log('Info logged successfully'))
        .catch(error => console.error('Error logging info:', error));
    };
}

using Xunit;
public class JsHelperServiceTests
{
    private readonly Mock<ILogger<JsHelperService>> _mockLogger;
    private readonly JsHelperService _jsHelperService;
    public JsHelperServiceTests()
    {
        _mockLogger = new Mock<ILogger<JsHelperService>>();
        _jsHelperService = new JsHelperService(_mockLogger.Object);
    }
    [Fact]
    public void WriteInfo_LogsMessage()
    {
        var message = "Test log message";
        _jsHelperService.WriteInfo(message);
        _mockLogger.Verify(logger => logger.LogInformation(message), Times.Once);
    }
}

Blazor JavaScript Interoperability: Beyond the Basics

For Blazor Server to construct powerful online applications, JavaScript and.NET integration is essential. But using Blazor to work with static services can be challenging, particularly when engaging with JavaScript. When calling.NET functions from JavaScript, the error "No call dispatcher has been set" frequently occurs. Blazor's JavaScript Interop, which depends on the call dispatcher to handle cross-environment calls, usually has this problem due to improper setup or missing configurations. To prevent such mistakes, one must comprehend how Blazor initializes and maintains its dispatcher.

Making sure the service that exposes.NET functions is correctly instantiated at application launch is one way to address this problem. The service is added as a singleton in Program.cs, so you know it will be there for the duration of the application. Given that static classes like JsHelperService aren't dependent on any particular component, this is particularly crucial when using them. Maintaining the service makes sure that JavaScript can call.NET methods continually without encountering problems related to lifecycles.

Verifying the existence of the DotNet object in the JavaScript environment is another crucial component. It is necessary for the window.DotNet object to load and be available before calling any.NET methods from JavaScript. Make sure that the Blazor.webassembly.js file initializes this object appropriately, or else errors such to the one below may occur. You can trace this object's availability by using JavaScript debuggers to monitor initialization.