Pass one of two possible claims to subjectnaminginfo - azure-ad-b2c

Question

I have a relying party for an azure b2c signup_signing custom policy where I want to output a few claims and set one of the claims, the email address, as the subjectnaminginfo. This works fine at a basic level, but there are two possible input claims which could be used as this email address and I want to pass through only one. In the snippet below, I'd like to use the "extensionAttribute11" claim as subject naming info IF it is present, and if it isn't present I'd like to use the email claim. How can I make this happen?
<TechnicalProfile Id="PolicyProfile">
<DisplayName>PolicyProfile</DisplayName>
<Protocol Name="SAML2" />
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="givenName" PartnerClaimType="givenName" />
<OutputClaim ClaimTypeReferenceId="surname" PartnerClaimType="surname" />
<OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="email" />
<OutputClaim ClaimTypeReferenceId="extension_7e93b79c37504f30a2345fa9ac315bb0_extensionAttribute11" PartnerClaimType="extensionAttribute11" />
</OutputClaims>
<SubjectNamingInfo ClaimType="email" ExcludeAsClaim="false"/>
</TechnicalProfile>
I've tried setting the PartnerClaimType for both claims to be "email" but that just errors.
I've tried looking into claims transformations but I can't find a way of doing what I want (essentially an "if null then... else..." logical statement)

Answer 1

You can use a conditional step in the user journey with a claims transformation technical profile.
So if the claim does not exist, the step is skipped.
The step only runs the claims transformation to copy the extension claim to email claim and outputs the new email claim.
Claims transformation:
<ClaimsTransformation Id="SetEmailFromExtension" TransformationMethod="CopyClaim">
<InputClaims>
<!-- Usually the app id is not mentioned for extensions? -->
<InputClaim ClaimTypeReferenceId="extension_extensionAttribute11"
TransformationClaimType="inputClaim" />
</InputClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="email" TransformationClaimType="outputClaim" />
</OutputClaims>
</ClaimsTransformation>
Technical profile:
<TechnicalProfile Id="Transform-SetEmailFromExtension">
<DisplayName>SetEmailFromExtension</DisplayName>
<Protocol Name="Proprietary"
Handler="Web.TPEngine.Providers.ClaimsTransformationProtocolProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
<InputClaims>
<InputClaim
ClaimTypeReferenceId="extension_extensionAttribute11" />
</InputClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="email" />
</OutputClaims>
<OutputClaimsTransformations>
<OutputClaimsTransformation ReferenceId="SetEmailFromExtension" />
</OutputClaimsTransformations>
<UseTechnicalProfileForSessionManagement ReferenceId="SM-Noop" />
</TechnicalProfile>
User journey step:
<OrchestrationStep Order="12" Type="ClaimsExchange">
<Preconditions>
<Precondition Type="ClaimsExist" ExecuteActionsIf="false">
<Value>extension_extensionAttribute11</Value>
<Action>SkipThisOrchestrationStep</Action>
</Precondition>
</Preconditions>
<ClaimsExchanges>
<ClaimsExchange Id="SetEmailFromExtension"
TechnicalProfileReferenceId="Transform-SetEmailFromExtension" />
</ClaimsExchanges>
</OrchestrationStep>