How to write unit test for a builder design pattern?

Question

I am using xunit for my unit testing and I am new to unit testing. I have a builder class like below,

public class UserBuilder
{
    private User _user = new User();

    public UserBuilder UserBuilder(IDataBase db) //IDatabase is available in a different Nuget package which I don't have source code access
    {
        this._user.db = db;
    }
    
    public UserBuilder SetName(string name)
    {
        this._user.name = name.ToLower();
    }

    public UserBuilder SetRole(string role)
    {
        this._user.role = role.ToUpper();
    }

   public void Validate()
   {
       if (this._user.name == null)
           throw  new InvalidDataException();
       if (this._user.db.contains(this._user.name)
           throw new InvalidDataException();
       ......
   }

   public User Build()
   {
      this.validate();
      this._user.db.persist();
      return this._user;
}

I would like to unit test the Build() , Validate() and SetName() but I don't have DB details with me and hence, not sure how to override contains and persist steps.

public class UserBuilderTest
{
    [Fact]
    public void Task_SetName_John()
    {
        // Arrange
        // How to Mock the IDatabase?
        var builder = new UserBuilder(FakeDatabase);

        // Act
        var actual = builder.Validate();

        // Assert
        Assert.Throws<InvalidDataException>(action);
    }   
}

Moreover, how can I check if the name is lower case after calling SetName() from the private _user field?

public class UserBuilderTest
{
    [Fact]
    public void Task_SetName_John()
    {
        // Arrange
        // How to Mock the IDatabase?
        var builder = new UserBuilder(FakeDatabase);
        var expected = 'john";

        // Act
        var actual = builder.SetName("John");

        // Assert
        Assert.Equals(expected, actual);
    }   
}

Answer 1

I've used the Moq library for this. https://github.com/moq/moq4

Usage would look something like this:

var mockDatabase = new Mock<IDataBase>();
// Do setups on the mock if necessary
// mockDatabase.Setup( m => m.SomeMethodOnTheInterface() ).Returns( new Something() );
var builder = new UserBuilder(mockDatabase.Object);