Validation phase - Unit Testing - how to write Unit test cases

Preparing a Unit test case document commonly referred as UTC is an important task in unit testing activity. Having a complete UTC with every possible test case leads to complete unit testing and thus gives an assurance of defect free unit at the end of unit testing stage.

While preparing unit test cases the following aspects should be kept in mind-

Expected functionality
Write test cases against each functionality that is expected to be provided from the unit being developed. It is important that user requirements should be traceable to functional specifications which should be traceable to program specifications which should be traceable to unit test cases. Maintaining such traceability ensures that the application fulfills user requirements.

Input Values
- Write test cases for each of the inputs accepted by the unit. Every input has certain validation rule associated with it. Write test cases to validate this rule.
- There can be cross-field validations in which one field is enabled depending upon input of another field. Test cases for these should not be missed.
- Write test cases for the minimum and maximum values of input.
- Variables that hold data have their value limits. In case of computed fields, it is very important to write test cases to arrive at an upper limit value of the variables.
- Write test cases to check the arithmetic expressions with all possible combinations of values.

Output Values
- Write test cases to generate scenarios which will produce all types of output values that are expected from unit.

Screen Layout
Screen/report layout must be tested against the requirements. It should ensure that pages and screens are consistent.

Path Coverage
A unit may have conditional processing which results in various paths, the control can traverse through. Test cases must be written for each of these paths.

Assumptions and Transactions
A unit may assume certain things for it to function. Test cases must be written to check that the unit reports error if such assumptions are not met.
In case of database applications, test cases should be written to ensure that transactions are properly designed and in no way inconsistent data gets saved in the database.

Abnormal terminations and Error messages
Test cases should be written to test the behavior of unit in case of abnormal termination.
Error messages should be short, precise and self explanatory. They should be properly phrased and free of grammatical mistakes.