Dear Payalorna
Following steps will help you:
Step 1: Filter All FamilySymbol of Structural Framing and Bind with Combobox Using Display Member & Value Member
a. Display Member Contains Symbol Name.
b. Value Member Contains Symbol ID.
Step 2: As Per Selection from Combobox. Get Value of Combobox which is ID of Symbol.
Step 3: Now Get Symbol from Document and Cast it into Symbol using doc.GetElement() As FamilySymbol
Step 4: Use this Symbol into Familyinstance Creation Method.
In case of Already Created instances.
a. you can use FamilySymbol fs = (Placed instance Familysymbol); fs.ChangeTypeId( Selected Family Symbol id From Combo box) Method.