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To convert List to Set in Salesforce, add some values to List datatype and use addAll() to add it to Set datatype.
Sample Code:
List < String > tempList = new List < String >();
Set < String > tempSet = new Set < String >();
tempList.add( 'One' );
tempList.add( 'Two' );
tempList.add( 'One' );
System.debug(
'tempList is' + tempList
);
tempSet.addAll( tempList );
System.debug(
'tempSet is' + tempSet
);
tempList is List datatype and tempSet is Set datatype.
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