Interface Conversion Panic — Type Assertion Failure

Why This Happens

When you assert an interface value to a concrete type with x.(int) and the actual value is not an int, Go panics. The safe way is to use the two-value form: val, ok := x.(int), where ok is false if the assertion fails instead of panicking.

The Problem

func process(val interface{}) {
    n := val.(int) // panics if val is not int
    fmt.Println(n * 2)
}

func main() {
    process("hello") // panic: interface conversion
}

The Fix

func process(val interface{}) {
    n, ok := val.(int)
    if !ok {
        fmt.Println("expected int, got something else")
        return
    }
    fmt.Println(n * 2)
}

Step-by-Step Fix

1. 1

   Identify the assertion

   Find the type assertion in your code and check what concrete type is actually stored in the interface.

2. 2

   Use the two-value form

   Change x.(T) to val, ok := x.(T) to prevent panics on wrong types.

3. 3

   Consider a type switch

   If the interface can hold multiple types, use a type switch to handle each case explicitly.