LoanUnderwriting

Loan Underwriting: a decision table with Bobcat

A small sample that shows how to use Bobcat to test a business rule with a table: several inputs per row, several expected outputs per row, and a verdict for every output cell.

There is no database, no messaging and no host here on purpose. The rule is one pure function, and the whole sample runs with nothing but the .NET SDK.

What it is meant to show

• A decision table as one specification. UnderwritingDecisionTable runs sixteen loan applications through the rule with TableRunner.Run. Ordinary parameters are the input columns, out parameters are output columns, and the returned value is the last output column. The run renders as one grid.
• The same idea as one specification per row. UnderwritingExamples puts the rows on an xUnit theory with [BobcatTheory] and [BobcatExamples]. Each row is its own test with its own verdict and identity, and the table is checked at build time.
• Failures that say which cell was wrong. A wrong row doesn't stop the table. Every output of every row is judged, and a failed cell reads expected 'Approve', got 'Decline'.
• Testing a business rule at the lowest layer that can prove it. The rule is a static method, so the specifications call it directly.

The rule

Underwriting.Decide() in LoanUnderwriting/Underwriting.cs takes a credit score, a debt to income percentage, an annual income and a loan amount, and answers with a decision (Approve, Refer or Decline), an interest rate and whether a guarantor is required.

1. A credit score under 580 is declined.
2. A debt to income over 43% is declined.
3. A loan bigger than half of annual income is referred to a person, unless the score is prime (740+).
4. Prime (740+) is approved at 6.25%, plus 0.75 when debt to income is over 36%, plus 0.50 for a large loan.
5. Standard (670 to 739) is approved at 7.50%, or referred when debt to income is over 36%.
6. Subprime (580 to 669) is approved at 9.75% with a guarantor, or declined when debt to income is over 36%.

Running it

You need the .NET 10 SDK.

dotnet test --project LoanUnderwriting.Specs

To see the rendered specifications, install the Bobcat tool and run through it:

dotnet tool install -g Bobcat.Console
bobcat run LoanUnderwriting.Specs --verbose
bobcat run LoanUnderwriting.Specs --html results.html    # one self-contained page

--verbose prints passing specifications too. Without it you only see the ones that failed.

Seeing it fail

Change one comparison in Underwriting.Decide() from > to >=:

var stretched = application.DebtToIncome >= StretchedDebtToIncome;

Rerun, and the two rows that sit exactly on the 36% boundary go red in the decision table, with every wrong output named. In UnderwritingExamples exactly one of the five rows fails and the other four still pass.

Layout

LoanUnderwriting/Underwriting.cs	the rule, as a pure function
LoanUnderwriting.Specs/UnderwritingDecisionTable.cs	one specification, one table
LoanUnderwriting.Specs/UnderwritingExamples.cs	one specification per row
global.json	puts dotnet test on Microsoft.Testing.Platform, which xUnit v3 4.x needs

See the Data Intensive Specifications tutorial in the Bobcat documentation for everything else a table can do.