Implementing Client-Side Typed Proxies

In this lesson, you will learn how to leverage C# interfaces to create strongly-typed SignalR proxies, replacing error-prone string-based method invocations with compile-time safety.

The Problem with Magic Strings

When working with SignalR, you typically invoke methods on the server using HubConnection.InvokeAsync("SendMessage", user, message). This approach is fragile; if you rename the method on the server or make a typo in the string, the compiler cannot warn you. You only discover the error at runtime when the server fails to find the target.

Defining a Typed Contract

To solve this, we define a common interface that both the Hub and the client proxy recognize. This interface acts as a "source of truth" for the communication contract.

public interface IChatHub
{
    Task SendMessage(string user, string message);
    Task JoinGroup(string groupName);
}

Implementing the Proxy

By using a typed proxy, we wrap the HubConnection to ensure every method call corresponds to our interface. While libraries can generate these automatically, at this stage, we implement the invocation pattern to map the interface to the underlying connection:

public class ChatClientProxy : IChatHub
{
    private readonly HubConnection _connection;

    public ChatClientProxy(HubConnection connection) => _connection = connection;

    public Task SendMessage(string user, string message) => 
        _connection.InvokeAsync(nameof(SendMessage), user, message);

    public Task JoinGroup(string groupName) => 
        _connection.InvokeAsync(nameof(JoinGroup), groupName);
}

Why This Matters

By using nameof(SendMessage), we gain refactoring support. If you rename the method in the IChatHub interface, the proxy class will immediately flag a build error if the nameof reference is broken. This pattern bridges the gap between the loose nature of WebSocket messaging and the strict expectations of the C# type system, ensuring that your client-side code stays in sync with your backend Hub implementation.

Key Takeaway: By binding your SignalR invocations to a shared interface, you transform brittle string-based communication into a type-safe contract that catches integration errors during compilation rather than at runtime.