TestVisible annotation allow test methods to access private or protected members of another class outside the test class. These members include methods, member variables, and inner classes. This annotation enables a more permissive access level for running tests only. This annotation doesn’t change the visibility of members if accessed by non-test classes.

With this annotation, you don’t have to change the access modifiers of your methods and member variables to public if you want to access them in a test method. For example, if a private member variable isn’t supposed to be exposed to external classes but it should be accessible by a test method, you can add the TestVisible annotation to the variable definition.

This example shows how to annotate a private class member variable and private method with TestVisible and how to access it in test class.

Sample Class:

public class TestVisibleExample {
    //Private member variable
    @TestVisible private static Integer recordNumber = 1;

    //Private method
    @TestVisible private static void updateRecord(String name) {
        // Do something
    }
}    

Test Class:

@isTest
private class TestVisibleExampleTest {
    @isTest static void test1() {
        // Access private variable annotated with TestVisible
        Integer i = TestVisibleExample.recordNumber;
        System.assertEquals(1, i);

        // Access private method annotated with TestVisible
        TestVisibleExample.updateRecord('RecordName');
        // Perform some verification
    }
}