public void Concat(string stringValues, string inputHeaderStringValues, string expectedOutput) { var request = new HttpRequestMessage(); foreach (var value in inputHeaderStringValues.Split(';')) { request.Headers.TryAddWithoutValidation("foo", value); } request.Headers.TryAddWithoutValidation("bar", inputHeaderStringValues.Split(';')); var headerStringValues = request.Headers.NonValidated["foo"]; var actualValues = RequestUtilities.Concat(stringValues?.Split(';'), headerStringValues); Assert.Equal(expectedOutput.Split(';'), actualValues); headerStringValues = request.Headers.NonValidated["bar"]; actualValues = RequestUtilities.Concat(stringValues?.Split(';'), headerStringValues); Assert.Equal(expectedOutput.Split(';'), actualValues); }