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Templating Parameter Double Lookup

In Tridion templating, double look-up refers to a technique of reading package values two or more times. This is probably most commonly used when reading TBB parameters.

For example, let's consider a TBB that takes a parameter called 'inputValue' of type String, which represents a TcmUri. In Template Builder, the template developer can specify a value for 'inputValue' in several ways:

  • hard-code an actual TcmUri, e.g. tcm:1-2;
  • use a package item name - e.g. package item named MyVariable, whose value is the TcmUri;
  • use a Dreamweaver Language Expression - e.g. Component.Fields.BannerImage that resolves to a TcmUri;

Hard-Coded Value
The look-up in the package is direct:

var value = package.GetValue("inputValue");

Package Item Name
I need to first look-up the package item, and then take its value. This is a form of double look-up:

var paramInputValue = package.GetValue("inputValue");
Item valueItem = package.GetByName(paramInputValue); // double look-up
var value = valueItem.GetAsString();

Alternatively, the package item name look-up can be done using Dreamweaver Language Expressions, see next. The two are thus equivalent.

Dreamweaver Language Expressions
You can also use a double look-up, much like the package item name:

var paramInputValue = package.GetValue("inputValue");
var value = package.GetValue(paramInputValue);

Why Stop at Double Look-Up?
Why not have triple look-up, or better yet N-look-up? Well, probably not very likely to use these cases, but technically, they are definitely possible.

var value = package.GetValue("inputValue");
String aux = package.GetValue(value);

while (aux != null) {
    value = aux;
    aux = package.GetValue(value);
}

Putting it All Together
string imageTcmUri = m_Package.GetValue("ImageTcmUri"); // direct look-up
if (String.IsNullOrEmpty(imageTcmUri))
{
     throw new Exception("Mandatory parameter ImageTcmUri cannot be empty");
}
else if (!TcmUri.IsValid(imageTcmUri)) // give second chance
{
     imageTcmUri = m_Package.GetValue(imageTcmUri); // double lookup
     if (String.IsNullOrEmpty(imageTcmUri) || !TcmUri.IsValid(imageTcmUri)) // fail
     {
          throw new Exception("Invalid parameter ImageTcmUri");
     }
}

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