Moq

// Arrange... DateTime? callbackDatum = null;

policyService.Setup(ps => ps.GetPolicyDetailsByPolicyNumberInternal(It.IsAny<GetPolicyDetailsByPolicyNumberInternalRequestType>(), It.IsAny<LogHerkomst>(), It.IsAny<object>()))

.Callback( (GetPolicyDetailsByPolicyNumberInternalRequestType g, LogHerkomst l, object o) => { callbackDatum = g.ReferenceDate; }) .Returns(policyDetailsResponse);

// Assert... callbackDatum.Should().Be(expectedPolicyChangeEffectDate);