Use DataSourceFixed to determine whether the DataSource property can be changed. By default, DataSourceFixed is false, allowing the DataSource property to be changed.  

When there is a chance that an asynchronous event might change the DataSource before an operation completes, set DataSourceFixed to true before performing the operation to ensure that the DataSource is constant throughout the operation. After the data operation is complete, DataSourceFixed can be set back to false. 

Set DataSourceFixed to prevent the DataSource of a data-aware object that owns the TDataLink from being changed once it is set. 

DataSource