Keep runtime resource lookups out of ARM parameter defaults

Source facts

Resource Manager settles parameter values before deployment operations begin. ARM parameter defaults therefore cannot use reference or list functions to retrieve runtime resource state. A default can use another parameter, but not a template variable. Expressions are supported for defaults, not for the other parameter properties. Microsoft Learn.

Applicability

Apply this review when a reusable template tries to discover an existing or newly deployed resource while establishing its inputs. Identify exactly where the expression appears; the question here is parameter evaluation, not whether the resource can be queried elsewhere.

DSE recommendation

DSE recommends separating caller-supplied inputs from values that must be discovered during resource processing. For each proposed default, record what it depends on and when that dependency becomes available. Where the caller must supply the value, make that obligation clear instead of hiding it behind an invalid default. Do not treat an added deployment dependency as a substitute for reviewing the parameter expression itself.

Verification

Use a small, nonproduction template to check the proposed default and a supported parameter-to-parameter alternative. Keep any runtime lookup out of the parameter definition during the comparison. Verify both the explicitly supplied and omitted-input paths, and retain validation diagnostics with the template revision. Confirm the resulting value matches the intended configuration before carrying the revised input design into a resource deployment.

Official references

Microsoft Learn. Source retrieved September 9, 2026.