Hi Himanshu,

The System.runAs method in Salesforce is used to run a block of code with a specified user's context. It allows you to temporarily switch to another user's permissions and record access level for testing or performing operations that require elevated privileges.
Here's an example to illustrate the use of System.runAs:
Let's say you have a Salesforce org with different user roles and profiles. You have implemented a complex Apex trigger that performs certain operations on an object when it is created. Now, you want to test whether the trigger logic works correctly for different user scenarios.
In this case, you can use System.runAs to simulate the behavior of different users while testing the trigger.

For ex.

@isTest
private class TriggerTestingExample {
    @isTest
    static void testTriggerLogic() {
        // Create a user with specific role and profile for testing
        Profile testProfile = [SELECT Id FROM Profile WHERE Name = 'Testing Profile' LIMIT 1];
        UserRole testRole = [SELECT Id FROM UserRole WHERE Name = 'Testing Role' LIMIT 1];
        User testUser = new User(
            Alias = 'testuser',
            Email = 'testuser@example.com',
            EmailEncodingKey = 'UTF-8',
            LastName = 'Testing',
            LanguageLocaleKey = 'en_US',
            LocaleSidKey = 'en_US',
            ProfileId = testProfile.Id,
            TimeZoneSidKey = 'America/Los_Angeles',
            UserName = 'testuser@example.com',
            UserRoleId = testRole.Id
        );
        insert testUser;

        // Create a test object
        MyCustomObject__c obj = new MyCustomObject__c(Name = 'Test Object');
        insert obj;

        // Perform the test scenario with different user context using System.runAs
        System.runAs(testUser) {
            // Perform actions that need to be executed as the test user
            // This can include creating, updating, or deleting records, etc.
            // For example, test the trigger logic on the newly created object
            // and assert the expected results
            Test.startTest();
            // Perform actions to test the trigger logic
            Test.stopTest();

            // Assert the expected results
            // For example, verify that the trigger has performed the expected operations
            System.assertEquals(...);
        }
    }
}

In this example, the System.runAs block is used to execute the code as the testUser. Any operations performed within the block, such as creating records, updating fields, or triggering the execution of a trigger, will run in the context of the testUser. This allows you to test the trigger logic and ensure it behaves correctly for different user scenarios.


If this helps please mark it as a best answer,
Thanks!