Type-State Builder Pattern

The type-state builder pattern uses interface progression to enforce compile-time safety, ensuring objects are constructed with all required properties in the correct order. Invalid states become impossible to represent.

Interface Progression

using System.Net;
using System.Runtime.CompilerServices;
using Microsoft.AspNetCore.Mvc;

namespace Shared.Types;

public interface IProblemBuilder<T>
{
    IProblemBuilderWithSource<T> WithSourceId(
        string sourceId);
}

public interface IProblemBuilderWithSource<T>
{
    IProblemBuilderWithTitle<T> WithTitle(string title);
}

public interface IProblemBuilderWithTitle<T>
{
    IProblemBuilderWithDetail<T> WithDetail(
        string detail);
}

public interface IProblemBuilderWithDetail<T>
{
    IProblemBuilderComplete<T> WithStatusCode(
        HttpStatusCode statusCode);
}

public interface IProblemBuilderComplete<T>
{
    ProblemDetailsResult<T> Create(
        [CallerMemberName] string callerMemberName = "");
}

Implementation

internal class ProblemBuilder<T> : 
    IProblemBuilder<T>, 
    IProblemBuilderWithSource<T>,
    IProblemBuilderWithTitle<T>, 
    IProblemBuilderWithDetail<T>, 
    IProblemBuilderComplete<T>
{
    private string? _sourceId;
    private string? _title;
    private string? _detail;
    private HttpStatusCode _statusCode;

    public IProblemBuilderWithSource<T> WithSourceId(
        string sourceId)
    {
        _sourceId = sourceId;
        return this;
    }

    public IProblemBuilderWithTitle<T> WithTitle(
        string title)
    {
        _title = title;
        return this;
    }

    public IProblemBuilderWithDetail<T> WithDetail(
        string detail)
    {
        _detail = detail;
        return this;
    }

    public IProblemBuilderComplete<T> WithStatusCode(
        HttpStatusCode statusCode)
    {
        _statusCode = statusCode;
        return this;
    }

    public ProblemDetailsResult<T> Create(
        [CallerMemberName] string callerMemberName = "")
    {
        return new ProblemDetailsResult<T>
        {
            SourceId = _sourceId!,
            StatusCode = _statusCode,
            ProblemDetails = new ProblemDetails
            {
                Title = _title!,
                Detail = _detail!,
                Status = (int)_statusCode
            },
            CallerMemberName = callerMemberName
        };
    }
}

SOLID Principles Demonstrated

Single Responsibility Principle: Each interface represents one step in the construction process
Open/Closed Principle: Extensible without modifying existing interfaces
Interface Segregation Principle: Clients depend only on interfaces they need at each step
Dependency Inversion Principle: Factory methods depend on abstractions, not concrete builder

Code Quality Design Help

Type-State Builder Pattern

Interface Progression

Implementation

SOLID Principles Demonstrated

See Also