How to throw DML Exception Manually

The easiest way to intentionally throw DML exception is insert record without setting required field.

• Exception Class and Built-In Exceptions

Quote:

Any problem with a DML statement, such as an insert statement missing a required field on a record.

To bring more clarity here is code spinet for your reference:

try{
    Account a = [Select id, name from account limit 1];
    a.name = '';
    update a;    
}
Catch(Exception e){
    system.debug('Exception ' + e);
}

There could be other ways to raise a DML Exception. For Example consider testAccount is a Account object in your test data. Now Below code will raise DML exception:

testAccount.Name='===================================================================================================Some string which is longer than 255 characters================================================================================================================';
update testAccount;

A DML Exception will be thrown and catch block would be covered. So you need to modify your test data accordingly.

Here is some sample code:

try{
    if(Test.isRunningTest() && updatedAccSt.size() > 0 && 
        updatedAccSt[0].Name='Exception Account'){
        updatedAccSt[0].Name='===================================================================================================Some string which is longer than 255 characters================================================================================================================';
    }
    update updatedAccSt;
}

Its easy.

throw new DMLException('My DML EXCEPTION');