Test Case Design

Test Case Design Techniques

When to Use This Skill

Use this skill when:

• Test Case Design tasks - Working on apply systematic test case design techniques including equivalence partitioning, boundary value analysis, decision tables, and state transition testing
• Planning or design - Need guidance on Test Case Design approaches
• Best practices - Want to follow established patterns and standards

Overview

Systematic test case design techniques ensure thorough coverage while minimizing test case count. These black-box techniques derive test cases from specifications without knowledge of internal implementation.

Equivalence Partitioning

Divide input data into equivalent classes where any value should produce the same behavior.

Process

1. Identify input conditions
2. Divide into valid and invalid partitions
3. Select one representative value from each partition
4. Create test cases for each partition

Example: Age Validation (18-65)

Test Cases:

• TC1: age = 10 → Reject (invalid below)
• TC2: age = 30 → Accept (valid)
• TC3: age = 70 → Reject (invalid above)

Multiple Input Partitions

Combine partitions systematically:

Input A: {Valid, Invalid}
Input B: {Valid, Invalid}

Combinations:
1. A-Valid, B-Valid → Expected: Success
2. A-Valid, B-Invalid → Expected: Error for B
3. A-Invalid, B-Valid → Expected: Error for A
4. A-Invalid, B-Invalid → Expected: Error for both

Boundary Value Analysis

Test at and around partition boundaries where defects commonly occur.

Process

1. Identify boundaries from equivalence partitions
2. Test at minimum, just below, just above, and maximum
3. Include special values (0, empty, null)

Example: Age Validation (18-65)

Extended Boundaries:

• 0 (edge case)
• -1 (negative)
• MAX_INT (overflow)
• null (missing)

.NET Example

public class AgeValidationTests
{
    [Theory]
    [InlineData(17, false)]  // Below minimum
    [InlineData(18, true)]   // At minimum
    [InlineData(19, true)]   // Above minimum
    [InlineData(40, true)]   // Normal
    [InlineData(64, true)]   // Below maximum
    [InlineData(65, true)]   // At maximum
    [InlineData(66, false)]  // Above maximum
    [InlineData(0, false)]   // Zero
    [InlineData(-1, false)]  // Negative
    public void ValidateAge_ReturnsExpected(int age, bool expected)
    {
        var result = _validator.IsValidAge(age);
        Assert.Equal(expected, result);
    }
}

Decision Table Testing

Test complex business rules with multiple conditions systematically.

Process

1. Identify conditions (inputs)
2. Identify actions (outputs)
3. Create table with all condition combinations
4. Simplify using "don't care" conditions

Example: Discount Calculation

Conditions:

• Is member? (Y/N)
• Order > $100? (Y/N)
• Has coupon? (Y/N)

Actions:

• Apply member discount (10%)
• Apply bulk discount (5%)
• Apply coupon discount (15%)

.NET Example

public class DiscountCalculationTests
{
    public static TheoryData<bool, decimal, bool, decimal> DiscountScenarios => new()
    {
        { true, 150m, true, 0.30m },   // R1: Member + Bulk + Coupon
        { true, 150m, false, 0.15m },  // R2: Member + Bulk
        { true, 50m, true, 0.25m },    // R3: Member + Coupon
        { true, 50m, false, 0.10m },   // R4: Member only
        { false, 150m, true, 0.20m },  // R5: Bulk + Coupon
        { false, 150m, false, 0.05m }, // R6: Bulk only
        { false, 50m, true, 0.15m },   // R7: Coupon only
        { false, 50m, false, 0.00m },  // R8: No discount
    };

    [Theory]
    [MemberData(nameof(DiscountScenarios))]
    public void CalculateDiscount_ReturnsExpected(
        bool isMember, decimal orderTotal, bool hasCoupon, decimal expectedDiscount)
    {
        var discount = _calculator.Calculate(isMember, orderTotal, hasCoupon);
        Assert.Equal(expectedDiscount, discount);
    }
}

State Transition Testing

Test systems with distinct states and transitions between them.

Process

1. Identify states
2. Identify valid transitions
3. Create state transition table/diagram
4. Design tests for each transition
5. Include invalid transition tests

Example: Order State Machine

         ┌──────────┐
         │  Draft   │
         └────┬─────┘
              │ submit()
              ▼
         ┌──────────┐
    ┌────│ Pending  │────┐
    │    └────┬─────┘    │
    │         │ approve()│ reject()
    │         ▼          ▼
    │    ┌──────────┐  ┌──────────┐
    │    │ Approved │  │ Rejected │
    │    └────┬─────┘  └──────────┘
    │         │ ship()
    │         ▼
    │    ┌──────────┐
    └────│ Shipped  │
         └────┬─────┘
              │ deliver()
              ▼
         ┌──────────┐
         │Delivered │
         └──────────┘

State Transition Table

.NET Example

public class OrderStateTests
{
    [Fact]
    public void Draft_Submit_TransitionsToPending()
    {
        var order = new Order { Status = OrderStatus.Draft };
        order.Submit();
        Assert.Equal(OrderStatus.Pending, order.Status);
    }

    [Fact]
    public void Draft_Approve_ThrowsInvalidStateException()
    {
        var order = new Order { Status = OrderStatus.Draft };
        Assert.Throws<InvalidOperationException>(() => order.Approve());
    }

    [Fact]
    public void Delivered_AnyAction_ThrowsFinalStateException()
    {
        var order = new Order { Status = OrderStatus.Delivered };
        Assert.Throws<InvalidOperationException>(() => order.Cancel());
    }
}

Pairwise Testing

Efficiently test combinations when full combinatorial testing is impractical.

Concept

Most defects are caused by interactions between 2 parameters. Testing all pairs covers most risks with fewer tests.

Example: Browser Compatibility

Parameters:

• Browser: Chrome, Firefox, Safari, Edge
• OS: Windows, macOS, Linux
• Version: Latest, Previous

Full combinations: 4 × 3 × 2 = 24 tests Pairwise coverage: 8-12 tests (covers all pairs)

Pairwise Test Set

Use tools like PICT, AllPairs, or online generators.

Error Guessing

Experience-based technique to identify likely defect areas.

Common Error Patterns

Error Guessing Checklist

• What if input is null?
• What if input is empty?
• What if input contains special characters?
• What if input exceeds maximum length?
• What if concurrent requests occur?
• What if external service fails?
• What if database connection drops?
• What if input is negative?
• What if list has duplicates?

Technique Selection Guide

Integration Points

Inputs from:

• Requirements → Test conditions
• test-strategy-planning skill → Coverage targets

Outputs to:

• acceptance-criteria-authoring skill → Scenario coverage
• Test automation → Test data

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