How to set a item field as ParameterExpression in Coveo

Question

When I try to create a custom rule and pass field name, I get Instance property 'textField' is not defined for type 'Sitecore.ContentSearch.SearchTypes.SearchResultItem' exception.

This is how my query expression is set.

    public Expression GetQueryExpression(ConditionContext p_Context)
    {
        Expression readNameProperty = Expression.Property(p_Context.ParameterExpression, "textField");
        Expression standardValuesConstant = Expression.Constant("somevalue");

        return Expression.Equal(readNameProperty, standardValuesConstant);
    }

Answer 1

By using Expression.Property(...) you are trying to access a Property with the name textField on the SearchResultItem, which corresponds to this:

SearchResultItem.textField

To get a field value you instead need to use the indexer on SearchResultItem which corresponds to this:

SearchResultItem['textField']

I don't know much about Coveo, but I found Creating a Custom Rule Condition for Coveo for Sitecore in their developer guide which explains it pretty well.

It's a bit complicated to do this, so in the guide they have made a method for just that: GetFieldIndexerExpression(...).

/// <summary>
/// Gets the <see cref="Expression"/> that will invoke the indexer for the field <paramref name="p_FieldName"/>.
/// </summary>
/// <remarks>The indexer must be called as a method, not as a property.</remarks>
/// <param name="p_Instance">The parameter instance of the Linq expression. This is the part of the Linq expression before the <c>=></c> symbol.
///   <example>item => item["fieldName"]</example>
/// </param>
/// <param name="p_FieldName">The field name to reference in the indexer.</param>
/// <returns>The <see cref="Expression"/> that will invoke the field indexer when executed.</returns>
Expression GetFieldIndexerExpression(ParameterExpression p_Instance, string p_FieldName)
{
    string indexerPropertyName = "Item";
    // By default the indexer corresponds to the "Item" property, but one can change it.
    // In order to find the name, if modified, we must search for DefaultMemberAttribute attribute.
    // Fortunately, only one indexer name can be defined.
    object[] attributes = p_Instance.Type.GetCustomAttributes(typeof(DefaultMemberAttribute), true);
    if (attributes.Any()) {
        indexerPropertyName = (attributes[0] as DefaultMemberAttribute).MemberName;
    }

    PropertyInfo indexerProperty = p_Instance.Type.GetProperty(indexerPropertyName,
                                                               new Type[] { typeof(string) });
    MethodInfo indexerGetMethod = indexerProperty.GetMethod;

    Expression callIndexerExpression = Expression.Call(p_Instance,
                                                       indexerGetMethod,
                                                       Expression.Constant(p_FieldName));
    return callIndexerExpression;
}

You then use it instead of Expression.Property(...) like this:

public Expression GetQueryExpression(ConditionContext p_Context)
{
    Expression readNameProperty = GetFieldIndexerExpression(p_Context.ParameterExpression, "textField");
    Expression standardValuesConstant = Expression.Constant("somevalue");

    return Expression.Equal(readNameProperty, standardValuesConstant);
}

With this it should now work correctly.