How to load the UniOffice license key?

Load it before you touch anything else in the library. UniOffice checks the license on the first operation, so a key loaded partway through main is loaded too late and every call before it fails. An init function is the conventional place.

Which call you use depends on which kind of license you have.

Both live in github.com/unidoc/unioffice/v2/common/license.

Metered API key

This is what the free tier gives you and what every example in the guides uses. Read the key from an environment variable rather than committing it:

package main

import (
    "os"

    "github.com/unidoc/unioffice/v2/common/license"
)

func init() {
    if err := license.SetMeteredKey(os.Getenv(`UNIDOC_LICENSE_API_KEY`)); err != nil {
        panic(err)
    }
}

The library does not read UNIDOC_LICENSE_API_KEY on its own. Your program reads it and passes the value in, which is why the os.Getenv call is in every example. Setting the variable without the SetMeteredKey call does nothing.

Offline license

An offline license is verified from its own signature and makes no outbound connection, which suits OEM distribution and networks that block egress. The customer name has to match the one embedded in the signed key.

const offlineLicenseKey = `
-----BEGIN UNIDOC LICENSE KEY-----
contents here.
-----END UNIDOC LICENSE KEY-----
`

func init() {
    if err := license.SetLicenseKey(offlineLicenseKey, `My Company`); err != nil {
        panic(err)
    }
}

Keep the key content exactly as issued, including the BEGIN and END lines and the line breaks between them. Reflowing it breaks the signature.

Letting the environment load an offline license

An offline license can be loaded without any code at all. If both of these are set, UniOffice reads the file at startup and applies it:

Both must be set; if either is missing the loader returns without doing anything. Failures to read or apply the file are logged rather than returned, so a wrong path leaves the program unlicensed with only a log line to say why.

This applies to offline licenses only. There is no equivalent for metered keys, which always go through SetMeteredKey.

A third variable, UNIDOC_LICENSE_DIR, sets where metered license information is cached. It defaults to your home directory, and matters mainly in containers or under service accounts that have no writable home.

Checking what was loaded

GetLicenseKey returns the active key, or nil if none was loaded:

lk := license.GetLicenseKey()
if lk == nil {
    fmt.Println("no license loaded")
    return
}
fmt.Printf("License: %s\n", lk.ToString())

If you are converting documents to PDF, one key is not enough: UniPDF keeps its own license state and needs the same key loaded separately. See Why does my license key fail when converting to PDF?