foo1.cs

var operand = dagOwnerElement.SourceElement is IIsExpression sourceIsExpression ? sourceIsExpression.Operand
  : dagOwnerElement.SourceElement is ISwitchStatement sourceSwitchStatement ? sourceSwitchStatement.GoverningExpression
  : dagOwnerElement.SourceElement is ISwitchExpression sourceSwitchExpression ? sourceSwitchExpression.GoverningExpression
  : null;

foo2.cs

var operand = dagOwnerElement.SourceElement switch {
  IIsExpression sourceIsExpression => sourceIsExpression.Operand,
  ISwitchStatement sourceSwitchStatement => sourceSwitchStatement.GoverningExpression,
  ISwitchExpression sourceSwitchExpression => sourceSwitchExpression.GoverningExpression,
  _ => null
};

foo3.cs

var operand = dagOwnerElement.SourceElement switch {
  IIsExpression x => x.Operand,
  ISwitchStatement x => x.GoverningExpression,
  ISwitchExpression x => x.GoverningExpression,
  _ => null
};