Builder Pattern

Builder Pattern in C#

The Builder Pattern helps construct complex objects step-by-step. It is useful when you want to prevent creating incomplete or invalid objects. For example, just like a car should not be created without a steering wheel, your object should not be created without required fields.

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Without Builder Pattern

When many parameters are required, constructors become difficult to maintain and easy to misuse.

public class User
{
    public string Name { get; }
    public int Age { get; }
    public string Email { get; }
    public string Address { get; }

    public User(string name, int age, string email, string address)
    {
        Name = name;
        Age = age;
        Email = email;
        Address = address;
    }
}

// Hard to remember parameter order
var user = new User("Vinodh", 30, "v@example.com", "Bangalore");

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With Builder Pattern (With Validation)

The Builder Pattern allows you to set values step-by-step and validate them before creating the final object.

public class User
{
    public string Name { get; private set; }
    public int Age { get; private set; }
    public string Email { get; private set; }
    public string Address { get; private set; }

    private User() { }

    public class Builder
    {
        private readonly User _user = new User();

        public Builder SetName(string name)
        {
            _user.Name = name;
            return this;
        }

        public Builder SetAge(int age)
        {
            _user.Age = age;
            return this;
        }

        public Builder SetEmail(string email)
        {
            _user.Email = email;
            return this;
        }

        public Builder SetAddress(string address)
        {
            _user.Address = address;
            return this;
        }

        public User Build()
        {
            // Validation logic – prevents invalid object creation
            if (string.IsNullOrWhiteSpace(_user.Name))
                throw new Exception("Name is required.");

            if (_user.Age <= 0)
                throw new Exception("Age must be a positive value.");

            if (string.IsNullOrWhiteSpace(_user.Email))
                throw new Exception("Email is required.");

            // Similar to: a car cannot be created without a steering wheel
            if (string.IsNullOrWhiteSpace(_user.Address))
                throw new Exception("Address cannot be empty.");

            return _user;
        }
    }
}

Usage

var user = new User.Builder()
    .SetName("Vinodh")
    .SetAge(30)
    .SetEmail("v@example.com")
    .SetAddress("Bangalore")
    .Build();

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Conclusion

The Builder Pattern prevents invalid object creation through step-by-step construction and validation. Just like a car must not be built without vital parts, your objects remain consistent and safe.