次のようにフィルター式を蓄積することで、タスクをよりエレガントに達成できます。
Expression<Func<PersonalInfo, bool>> filterExperssion = PredicateExtensions.True<PersonalInfo>();
if (!String.IsNullOrEmpty(firstName))
filterExperssion = filterExperssion.And(p => p.FirstName == firstName);
if (!String.IsNullOrEmpty(lastName))
......
述語を作成するには、この小さなクラスヘルパーが必要です
public static class PredicateExtensions
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
}
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1,
Expression<Func<T, bool>> expression2)
{
var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
}
}