As part of one of requirements in our application we had to appended a FlowDocument objects to another FlowDocument object. The requirement looked very simple and I tried the following which loops through the Blocks of source FlowDocument and appends them to target FlowDocument object.

sourceDocument = (FlowDocument)XamlReader.Load(memoryStream);
                    //
foreach (Block aBlock in sourceDocument.Blocks)
{
    targetDocument.Blocks.Add(aBlock);
}

But the above code broke with this exception

An exception of type ‘System.InvalidOperationException’ occurred and was caught. Collection was modified; enumeration operation may not execute.

Not sure the reason why the exception is raising when we loop through the Blocks of FlowDocument and add them to the another FlowDocument. But i resolved the issue by converting the Blocks of source object to List  and then adding them to the target FlowDocument. Here is the code that worked

//
sourceDocument = (FlowDocument)XamlReader.Load(memoryStream);
//
List<Block> flowDocumentBlocks = new List<Block>(sourceDocument.Blocks);

// 
foreach (Block aBlock in flowDocumentBlocks)
{
    targetDocument.Blocks.Add(aBlock);
}

Hope this piece of code helps you.