Skip to content

Boolean validators

Boolean parameters in Revit are tri-state: Yes, No, and Unset. That matters. A lot.

Boolean — Is

Used for

A Boolean validator checks only for:

true or false

Nothing else.

How it works

  • Reads the boolean value from the filter output
  • Compares it to the expected boolean state

What this means in practice

When you use a Boolean validator, you are asserting:

  • The parameter exists
  • The parameter is of boolean type
  • The value resolves to true or false

That’s it.

What a Boolean validator does not interpret

A Boolean validator does not understand:

  • "Yes" / "No" as text
  • 1 / 0 as numbers
  • Empty values
  • Missing parameters

Those are separate decisions and must be handled explicitly with:

  • Property validators (exist / not exist)
  • Null validators (should be null / not null)

Important Revit-specific reality (state this clearly)

In Revit, a Boolean parameter can be:

  • True
  • False
  • Unset

But for the Boolean validator:

  • Only true and false are valid inputs
  • Unset is not true and not false → it must be handled before or around the Boolean check

This ties directly back to your checklist:

  1. Does the parameter exist?
  2. Is it set?
  3. Only then: is it true or false?

Boolean — Is not

Is not Used for

Ensuring a boolean parameter is not set to a specific value.

Is not How it works

  • Reads the boolean value
  • Fails if it equals the forbidden value

What it will find

  • The opposite boolean value
  • Unset values (depending on context)

What it will not find

  • Missing parameters
  • Implicit false assumptions

⚠️ Revit Boolean pitfall — Unset is not False

In Revit, a Boolean parameter added ad Project Parameter has three possible states:

  • True
  • False
  • Unset

This is not theoretical — it happens in real models.

Why this matters for validation

A Boolean validator only evaluates:

  • true
  • false

If a Boolean parameter is Unset:

  • It is not true
  • It is not false
  • It does not automatically fail
  • It does not automatically pass

From the validator’s perspective, the value is simply not usable.

In Revit, for a Yes/No parameter:

  • Checked → True
  • Unchecked → False
  • Greyed out → Unset (no value assigned)

Two elements can both appear “not checked” in a report, while their data meaning differs:

  • False → explicitly unchecked
  • Unset → never set, hasValue = false

If a parameter is required, a greyed-out (Unset) value must fail — even though it is not true.

Correct validation pattern for mandatory boolean parameters

  1. Check that the parameter exists (property validator or $exists)
  2. Check that hasValue = true
  3. Then validate the boolean value as required

✅ Decide explicitly what Unset means for this rule. ❌ Never treat Unset as False.