| name | vfp-conventions |
| description | Visual FoxPro coding conventions and patterns. Use when writing or reviewing VFP code, understanding naming conventions, variable prefixes, or VFP-specific patterns. |
| allowed-tools | Read, Grep, Glob |
VFP Coding Conventions
Naming Conventions
Database Field Prefixes (CRITICAL!)
| Prefix | SQL Type | VFP Type | Example |
|---|
id* | INT IDENTITY/INT | Integer | idperson, idclient |
d* | DATE | Date | dbirthdate, ddeadline |
t* | DATETIME | DateTime | tlogon, tcreated |
n* | NUMERIC | Numeric | npercent, nprice |
c* | VARCHAR/CHAR | Character | cname, ccode |
l* | BIT | Logical | lactive, lbillable |
Variable Prefixes
| Prefix | Scope | Example |
|---|
gl_* | Global (PUBLIC) | gl_nSQLHandle, gl_cUser |
gll_* | Global saved to .mem file | gll_cExportPath |
m.* | Memory variables (Scatter) | m.cname, m.ddate |
lc* | Local Character | lcSQL, lcWhere |
ln* | Local Numeric | lnTotal, lnCount |
ld* | Local Date | ldStart, ldEnd |
ll* | Local Logical | llSuccess, llFound |
la* | Local Array | laResults, laFiles |
lo* | Local Object | loForm, loException |
Function Prefixes
| Prefix | Purpose | Example |
|---|
sql_* | SQL Server helpers | sql_exec(), sql_update() |
log_* | Logging functions | log_error(), log_info() |
get_* | Data retrieval | get_client(), get_settings() |
set_* | Data modification | set_status(), set_config() |
Code Patterns
1. VFP Does NOT Have ELSEIF!
*-- CORRECT - Use DO CASE
DO CASE
CASE nResult > 0
RETURN .T.
CASE nResult = 0
RETURN .T.
OTHERWISE
RETURN .F.
ENDCASE
*-- WRONG - VFP has no ELSEIF!
IF nResult > 0
RETURN .T.
ELSEIF nResult = 0 && COMPILE ERROR!
RETURN .T.
ENDIF
2. SCATTER for Read, REPLACE for Write (NOT GATHER!)
*-- CORRECT - Scatter to read, REPLACE to write specific fields
SELECT clients
Scatter Memvar
m.cname = "New Name"
REPLACE cname WITH m.cname
TABLEUPDATE(.T.)
*-- WRONG - Gather sends ALL fields (risky!)
Scatter Memvar
m.cname = "New Name"
Gather Memvar && May overwrite fields you didn't intend to change!
3. File Operations - Use STRTOFILE
*-- CORRECT - STRTOFILE for append
=STRTOFILE(lcText + CHR(13)+CHR(10), lcFile, .T.)
*-- WRONG - Fopen/Fputs/Fclose is fragile
lnHandle = Fopen(lcFile)
=Fputs(lnHandle, lcText)
=Fclose(lnHandle)
4. NULL Handling
*-- CORRECT - Check type before comparison
IF VARTYPE(ddeadline) = 'D' AND ddeadline < DATE()
lcStatus = "Overdue"
ENDIF
*-- WRONG - Will crash if field is NULL
IF ddeadline < DATE() && Error if NULL!
lcStatus = "Overdue"
ENDIF
5. Date Comparisons for "Overdue"
*-- CORRECT - < for strictly overdue (excludes today)
IIF(ddeadline < DATE(), "OVERDUE", "OK")
*-- WRONG - <= includes today (not overdue yet!)
IIF(ddeadline <= DATE(), "OVERDUE", "OK")
6. Error Handling with TRY/CATCH
TRY
*-- Risky operation
USE sometable EXCLUSIVE
*-- Process data
CATCH TO loEx
*-- Handle error
log_error(loEx.Message + " in " + loEx.Procedure)
MESSAGEBOX("Error: " + loEx.Message, 16, "Error")
FINALLY
*-- Cleanup (always runs)
USE IN SELECT("sometable")
ENDTRY
7. Safe Object Property Access
*-- CORRECT - Check object exists first
IF VARTYPE(loObject) = 'O' AND !ISNULL(loObject)
lcValue = loObject.Property
ENDIF
*-- WRONG - Will crash if object is NULL or doesn't exist
lcValue = loObject.Property
Common Errors
| Error | Cause | Solution |
|---|
| "Unrecognized command verb" | Used ELSEIF | Replace with DO CASE |
| "Operator/operand mismatch" | NULL comparison | Use VARTYPE() first |
| "Variable not found" | Missing m. prefix | Use m.varname with Scatter |
| "Property is read-only" | Modifying .Name | Some properties can't change at runtime |
Best Practices
- Always use m. prefix with SCATTER MEMVAR variables
- Prefer local variables (lc*, ln*) over public (gl_*)
- Close cursors explicitly - don't rely on scope cleanup
- Use TABLEUPDATE() after modifying buffered cursors
- Handle NULL values explicitly in comparisons
- Use TRY/CATCH for operations that might fail
- Log errors to help with debugging
Quick Reference
*-- String functions
ALLTRIM(string) && Remove leading/trailing spaces
UPPER(string) && Convert to uppercase
SUBSTR(string,n,m) && Extract substring
AT(search, string) && Find position
*-- Date functions
DATE() && Current date
DATETIME() && Current datetime
DTOC(date) && Date to character
CTOD(string) && Character to date
DTOS(date) && Date to string (YYYYMMDD)
*-- Type checking
VARTYPE(var) && Returns type letter (C,N,D,L,O,U,X)
TYPE("var") && Returns type (evaluates expression)
ISNULL(var) && Check if NULL
EMPTY(var) && Check if empty
*-- Array functions
ALINES(array, string, .T., CHR(13)+CHR(10)) && Split string to array
ALEN(array, 1) && Number of rows
ASCAN(array, value) && Find value in array
Resources
