Read SelLength to determine the length of the selected text in characters. Set SelLength to change the selection to consist of the first SelLength characters starting at SelStart. 

If SelectMode is msColumn, SelLength will be 0. Therefore do not use SelLength to determine presence of the selected block, instead use HaveSelection method.

Setting SelLength to a value greater than the number of characters from SelStart to the end of the text results in the selection of all characters from SelStart to the end of the text. Reading SelLength immediately after setting it to a value greater than the number of available characters returns the number of characters actually selected, not the value that was just set.

TCustomSyntaxMemo Properties