Response Value Handlers

Command handlers can return values that need to be processed by the command pipeline. The Application Model provides a flexible system for handling these return values through Response Value Handlers.

Automatic Response Handling

When a command handler returns a value, the command pipeline follows this logic:

Check for Value Handlers: The pipeline first checks if any registered response value handlers can handle the returned value using their CanHandle method
Process with Handler: If a value handler can handle the value, it processes the value and returns a CommandResult
Automatic Response Creation: If no value handler can handle the value, the pipeline automatically creates a CommandResult<T> with the returned value as the response

This means that command handlers can return any type of value, and it will either be processed by a specific handler or automatically become the command response.

Built-in Value Handlers

Out-of-the-box the Cratis ApplicationModel comes with the following value handlers:

Type	Description
ValidationResultResponseValueHandler	Responds to Cratis ValidationResult object and adds it to the command result

Command Handler Return Patterns

Single Value Return

public class CreateUserCommandHandler : ICommandHandler<CreateUserCommand>
{
    public Task<object?> Handle(CommandContext context)
    {
        var userId = Guid.NewGuid();
        // This will automatically create CommandResult<Guid> with userId as response
        return Task.FromResult<object?>(userId);
    }
}

OneOf Return

public class CreateUserCommandHandler : ICommandHandler<CreateUserCommand>
{
    public Task<object?> Handle(CommandContext context)
    {
        var result = OneOf<UserId, ValidationResult>.FromT0(new UserId(Guid.NewGuid()));
        // If no handler can process UserId, it becomes CommandResult<UserId>
        // If ValidationResultResponseValueHandler processes ValidationResult, it affects the command result
        return Task.FromResult<object?>(result);
    }
}

Tuple Return

public class CreateUserCommandHandler : ICommandHandler<CreateUserCommand>
{
    public Task<object?> Handle(CommandContext context)
    {
        var userId = new UserId(Guid.NewGuid());
        var auditInfo = new AuditInfo(DateTime.UtcNow, "system");
        
        // Each value is checked against available handlers
        // If no handler processes userId, it becomes the response
        // If a handler processes auditInfo, it affects the command result
        return Task.FromResult<object?>((userId, auditInfo));
    }
}

Creating Custom Value Handlers

You can create custom response value handlers by implementing the ICommandResponseValueHandler interface:

public class AuditInfoResponseValueHandler : ICommandResponseValueHandler
{
    public bool CanHandle(CommandContext commandContext, object value)
    {
        return value is AuditInfo;
    }

    public Task<CommandResult> Handle(CommandContext commandContext, object value)
    {
        var auditInfo = (AuditInfo)value;
        
        // Perform audit logging
        LogAuditEvent(auditInfo);
        
        // Return success - this doesn't affect the command response
        return Task.FromResult(CommandResult.Success(commandContext.CorrelationId));
    }
}

The Application Model will automatically discover and register custom value handlers in the command pipeline.

Response Object Availability

When implementing a command response value handler, the CommandContext.Response property contains the response object returned by the command handler, if any. This property can be null in the following scenarios:

The command handler didn't return anything (void method)
The command handler returned null
The command handler returned a single value that has no corresponding value handler (in which case that value becomes the automatic response)

Tuple Processing Behavior

When a command handler returns a tuple, the command pipeline intelligently processes each value:

Each value is evaluated against all available response value handlers using their CanHandle method
Values with handlers are processed by their respective response value handlers
Values without handlers are considered potential response values:
- If exactly one value has no handler, it becomes the response (available in CommandContext.Response)
- If multiple values have no handlers, a MultipleUnhandledTupleValuesException is thrown
- If all values have handlers, no response is set (CommandContext.Response remains null)

OneOf Processing Behavior

When a command handler returns a OneOf<T1, T2, ...> value:

The inner value is extracted from the OneOf wrapper
Value handlers are checked using the CanHandle method on the inner value
If a handler can process it, the handler processes the value
If no handler can process it, the inner value automatically becomes a CommandResult<T> response

Key Benefits

Zero Configuration: Values without handlers automatically become responses
Flexible Processing: Custom handlers can perform side effects (logging, notifications, etc.)
Type Safety: Automatic responses are properly typed as CommandResult<T>
Backward Compatibility: Existing value handlers continue to work as before

Example Implementation

public class MyResponseValueHandler : ICommandResponseValueHandler
{
    public bool CanHandle(CommandContext commandContext, object value)
    {
        // The commandContext.Response can be null here
        return value is MyValueType;
    }

    public Task<CommandResult> Handle(CommandContext commandContext, object value)
    {
        // Access the response if available
        var response = commandContext.Response; // This can be null
        
        if (response is not null)
        {
            // Handle cases where the command returned a response
            // This typically happens when the command returns a tuple with unhandled values
        }
        
        // Process the value that was returned by the command
        var myValue = (MyValueType)value;
        
        // Perform side effects (logging, notifications, etc.)
        ProcessValue(myValue);
        
        return Task.FromResult(CommandResult.Success(commandContext.CorrelationId));
    }
}

Migration Notes

From Previous Versions

If you were previously relying on the requirement that all returned values must have corresponding value handlers, this is no longer necessary. Values without handlers will now automatically become command responses, which provides:

Simplified Development: No need to create handlers for simple response values
Better Developer Experience: Commands can return domain objects directly
Reduced Boilerplate: Less code needed for basic response scenarios

Backward Compatibility

All existing value handlers continue to work exactly as before. The new automatic response creation only applies to values that have no corresponding handlers, ensuring full backward compatibility.

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