在Freemarker中包含带变量的模板(Include template with variable in Freemarker)

 取决于你喜欢linq查询语法的多少，可以直接使用扩展方法： 
 
var item = Items.First(i => i.Id == 123);
 
 并且如果您不想在列表为空时抛出错误，请使用FirstOrDefault返回元素类型的默认值（引用类型为null ）： 
 
var item = Items.FirstOrDefault(i => i.Id == 123);

if (item != null)
{
    // found it
}
 
 也可以使用Single()和SingleOrDefault() ，但是如果您正在从数据库读取或已经保证唯一性的内容，那么我不用担心，因为它必须扫描列表以查看是否有重复和抛出。 First()和FirstOrDefault()在第一个匹配上停止，所以它们更有效率。 
 
 在First()和Single()系列中，这里是他们抛出的地方： 
 
 
 
 First() - throws如果空/未找到，不抛出，如果重复 
 
 
 FirstOrDefault() - 如果空/未找到，则返回默认值，如果重复则不返回 
 
 
 Single() - throws if empty / not found，throws if duplicate exists 
 
 
 SingleOrDefault() - 如果空/未找到则返回默认值，如果存在重复则抛出 
 

Depends how much you like the linq query syntax, you can use the extension methods directly like:
 
var item = Items.First(i => i.Id == 123);
 
And if you don't want to throw an error if the list is empty, use FirstOrDefault which returns the default value for the element type (null for reference types):
 
var item = Items.FirstOrDefault(i => i.Id == 123);

if (item != null)
{
    // found it
}
 
Single() and SingleOrDefault() can also be used, but if you are reading from a database or something that already guarantees uniqueness I wouldn't bother as it has to scan the list to see if there's any duplicates and throws. First() and FirstOrDefault() stop on the first match, so they are more efficient.
 
Of the First() and Single() family, here's where they throw:
 
 
 
First() - throws if empty/not found, does not throw if duplicate
 
 
FirstOrDefault() - returns default if empty/not found, does not throw if duplicate
 
 
Single() - throws if empty/not found, throws if duplicate exists
 
 
SingleOrDefault() - returns default if empty/not found, throws if duplicate exists