Set the Text property to assign the value of the parameter to a string without changing the DataType. Unlike the AsString property, which sets the value to a string and changes the DataType, setting the text property converts the string to the DataType of the parameter, and sets the value accordingly. Thus, use AsString to treat the parameter as representing the value of a string field. Use Text instead when assigning a value that is in string form, when making no assumptions about the field type. For example, Text is useful for assigning user data that was input using an edit control. 

Reading the Text property is the same as reading the AsString property. 

AsString 

DataType 

TStringField