You can convert the expression that is returned from the editor. For example if your returned expression is 'decimalExpr', then you can convert it with a code similar to this one:
var newBody = Expression.Convert(decimalExpr.Body, typeof(double?));
var expr2 = Expression.Lambda(newBody, decimalExpr.Parameters) as Expression<
var func = expr2.Compile();
I hope that this is helpful.
All the best,
the Telerik team
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