/// <summary>
/// Returns a new delegate that safely gets the result of the specified expression
/// fragments.
/// </summary>
/// <typeparam name="TSource">The instance type.</typeparam>
/// <typeparam name="TResult">The result type.</typeparam>
/// <param name="expressionFragments">The expression fragments, which will be processed in the order as specified.</param>
/// <returns>The result or the default value of the result type, if one or more of the referenced members in the expression are <c>null</c>.</returns>
private static Func <TSource, TResult> CreateSafeGetDelegate <TSource, TResult>(IList <MemberExpression> expressionFragments)
{
List <Expression> targetFragments = new List <Expression>();
MemberExpression lastSourceFragment = expressionFragments.Last();
ParameterExpression instanceParameter = Expression.Variable(typeof(object), "instance");
ParameterExpression resultVariable = Expression.Variable(typeof(TResult), "result");
LabelTarget useDefaultLabel = Expression.Label("useDefault");
LabelTarget returnLabel = Expression.Label("return");
foreach (MemberExpression sourceFragment in expressionFragments)
{
MemberInfo memberInfo = (MemberInfo)sourceFragment.Member;
MemberExpression member = ConstantExpression.MakeMemberAccess(
Expression.Convert(instanceParameter, memberInfo.DeclaringType),
memberInfo);
if (sourceFragment != lastSourceFragment)
{
// Iterate into the member.
targetFragments.Add(
Expression.Assign(
instanceParameter,
Expression.Convert(member, typeof(object))));
// Value types cannot represent a null reference.
if (sourceFragment.Type.IsClass)
{
targetFragments.Add(
Expression.IfThen(
Expression.Equal(instanceParameter, Expression.Constant(null)),
Expression.Goto(useDefaultLabel)));
}
}
else
{
// The value of the final member is the actual result.
targetFragments.Add(
Expression.Assign(
resultVariable,
Expression.Convert(member, typeof(TResult))));
}
}
targetFragments.Add(Expression.Goto(returnLabel));
targetFragments.Add(Expression.Label(useDefaultLabel));
targetFragments.Add(Expression.Assign(resultVariable, Expression.Default(typeof(TResult))));
targetFragments.Add(Expression.Label(returnLabel));
targetFragments.Add(resultVariable);
Expression finalExpression = Expression.Block(new[] { resultVariable }, targetFragments);
return(Expression.Lambda <Func <TSource, TResult> >(finalExpression, instanceParameter).Compile());
}
