Crap Score

CRAP Score Analysis

Calculate CRAP (Change Risk Anti-Patterns) scores for .NET methods to identify code that is both complex and undertested.

Background

The CRAP score combines cyclomatic complexity and code coverage into a single metric:

$$\text{CRAP}(m) = \text{comp}(m)^2 \times (1 - \text{cov}(m))^3 + \text{comp}(m)$$

Where:

• $\text{comp}(m)$ = cyclomatic complexity of method $m$
• $\text{cov}(m)$ = code coverage ratio (0.0 to 1.0) of method $m$

A method with 100% coverage has CRAP = complexity (the minimum). A method with 0% coverage has CRAP = complexity^2 + complexity.

When to Use

• User wants to assess which methods are risky due to low coverage and high complexity
• User asks for CRAP score of specific methods, classes, or files
• User wants to prioritize which code to test next
• User wants to evaluate test quality beyond simple coverage percentages

When Not to Use

• User just wants to run tests (use run-tests skill)
• User wants to write new tests (use writing-mstest-tests skill or general coding assistance)
• User only wants a coverage percentage without complexity analysis

Inputs

Workflow

Step 1: Collect code coverage data

If no coverage data exists yet (no Cobertura XML available), always run `dotnet test` with coverage collection first and mention the exact command in your response. Do not skip this step -- CRAP scores require coverage data.

Check the test project's .csproj for the coverage package, then run the appropriate command:

Step 2: Compute cyclomatic complexity

Analyze the target source files to determine cyclomatic complexity per method. Count the following decision points (each adds 1 to the base complexity of 1):

Base complexity is 1 for every method. Each decision point adds 1.

When analyzing, read the source file and count these constructs per method. Report the breakdown.

Step 3: Extract per-method coverage from Cobertura XML

Parse the Cobertura XML to find each method's line-rate attribute under the target <class> element. If line-rate is not available at method level, compute it from the <lines> elements:

$$\text{cov}(m) = \frac{\text{lines with hits} > 0}{\text{total lines}}$$

Method names in Cobertura may differ from source (async methods, lambdas). Match by line ranges when names don't align.

Step 4: Calculate CRAP scores

For each method in scope, apply the formula:

$$\text{CRAP}(m) = \text{comp}(m)^2 \times (1 - \text{cov}(m))^3 + \text{comp}(m)$$

Step 5: Present results

Present a sorted table (highest CRAP first):

| Method                          | Complexity | Coverage | CRAP Score | Risk     |
|---------------------------------|------------|----------|------------|----------|
| OrderService.ProcessOrder       | 12         | 45%      | 28.4       | High     |
| OrderService.ValidateItems      | 8          | 90%      | 8.1        | Moderate |
| OrderService.CalculateTotal     | 3          | 100%     | 3.0        | Low      |

Include:

• Summary: total methods analyzed, how many in each risk category
• Top offenders: methods with CRAP > 30, with specific recommendations
• Quick wins: methods with high complexity but where small coverage improvements would drop the score significantly

Step 6: Provide actionable recommendations

For high-CRAP methods, suggest one or both:

1. Add tests -- identify uncovered branches and suggest specific test cases 2. Reduce complexity -- suggest extract-method refactoring for deeply nested logic

Calculate the coverage needed to bring a method below a CRAP threshold of 15:

$$\text{cov}_{\text{needed}} = 1 - \left(\frac{15 - \text{comp}}{\text{comp}^2}\right)^{1/3}$$

This formula only applies when comp < 15. When comp >= 15, the minimum possible CRAP score (at 100% coverage) is comp itself, which already meets or exceeds the threshold. In that case, coverage alone cannot bring the CRAP score below the threshold -- the method must be refactored to reduce its cyclomatic complexity first.

Report this as: "To bring ProcessOrder (complexity 12) below CRAP 15, increase coverage from 45% to at least 72%." For methods where complexity alone exceeds the threshold, report: "ComplexMethod (complexity 18) cannot reach CRAP < 15 through testing alone -- reduce complexity by extracting sub-methods."

Validation

• Verify that coverage data was collected successfully (Cobertura XML exists and contains data)
• Cross-check that method names in coverage data match the source code
• Confirm CRAP scores by spot-checking the formula on one method manually
• Ensure a 100%-covered method's CRAP equals its complexity exactly

Common Pitfalls

• Stale coverage data: Always regenerate coverage before computing CRAP scores. Old coverage files will produce misleading results.
• Method name mismatches: Cobertura XML may use mangled/compiler-generated names for async methods, lambdas, or local functions. Match by line ranges when names don't align.
• Generated code: Exclude auto-generated files (e.g., *.Designer.cs, *.g.cs) from analysis unless explicitly requested.