gistfile1.cs

public class StringBuilder2Test
  {
		[Fact]
		public void ToString_IsEmpty_ReturnEmptyString()
		{
			var sb = new StringBuilder2();
			Assert.Equal("", sb.ToString());
		}

		[Fact]
		public void ToString_HasAValue_RetunsTheValue()
		{
			var sb = new StringBuilder2();

			sb.AppendText("hello");
			Assert.Equal("hello", sb.ToString());
		}

		[Fact]
		public void ToString_AppendMultiple_RetunsMultipleValuesAppended()
		{
			var sb = new StringBuilder2();

			sb.AppendText("hello");
			sb.AppendText(" world");

			Assert.Equal("hello world", sb.ToString());
		}

		[Fact]
		public void ToString_AppendParameterAtTall_ReturnCorrectValue()
		{
			var sb = new StringBuilder2();

			sb.AppendText("hello world {0}", "Peter");
			
			Assert.Equal("hello world Peter", sb.ToString());
		}

		[Fact]
		public void ToString_AppendParameterAtHead_ReturnCorrectValue()
		{
			var sb = new StringBuilder2();

			sb.AppendText("{0} hello world", "Peter");

			Assert.Equal("Peter hello world", sb.ToString());
		}

		[Fact]
		public void ToString_AppendParameterAtMiddle_ReturnCorrectValue()
		{
			var sb = new StringBuilder2();

			sb.AppendText("hello {0} world", "Peter");

			Assert.Equal("hello Peter world", sb.ToString());
		}

		[Fact]
		public void ToString_AppendTwoParameterAtTail_ReturnCorrectValue()
		{
			var sb = new StringBuilder2();
			sb.AppendText("hello world {0} {1}", "Peter", "Tran");

			Assert.Equal("hello world Peter Tran", sb.ToString());
		}

		[Fact]
		public void ToString_AppendThreeParameterAtTail_ReturnCorrectValue()
		{
			var sb = new StringBuilder2();
			sb.AppendText("hello world {0} {1} {2}", "Peter", "Tran", "Foo");

			Assert.Equal("hello world Peter Tran Foo", sb.ToString());
		}

	}

gistfile2.cs

public class StringBuilder2
  {
		private string result = string.Empty;

		public override string ToString()
		{
			return this.result;
		}

		internal void AppendText(string value, params string[] parameters)
		{
			this.result += value;

			for (int i = 0; i < parameters.Length; i++)
			{
				this.result = this.result.Replace("{" + i + "}", parameters[i]);
			}

		}
	}