[英]How to create a System.Linq.Expressions.Expression for Like?
我从这个源创建了一个可过滤的BindingList。 它很棒:
list.Filter("Customer == 'Name'");
应该做什么。 内部!=
像解析器一样工作,将表达式==
或!=
转换为System.Linq.Expressions.Expression
。 在这种情况下, ==
成为System.Linq.Expressions.Expression.Equal
。
不幸的是System.Linq.Expressions.Expression
不包含like运算符,我不知道如何解决这个问题。
初始代码如下所示:
private static Dictionary<String, Func<Expression, Expression, Expression>>
binaryOpFactory = new Dictionary<String, Func<Expression, Expression, Expression>>();
static Init() {
binaryOpFactory.Add("==", Expression.Equal);
binaryOpFactory.Add(">", Expression.GreaterThan);
binaryOpFactory.Add("<", Expression.LessThan);
binaryOpFactory.Add(">=", Expression.GreaterThanOrEqual);
binaryOpFactory.Add("<=", Expression.LessThanOrEqual);
binaryOpFactory.Add("!=", Expression.NotEqual);
binaryOpFactory.Add("&&", Expression.And);
binaryOpFactory.Add("||", Expression.Or);
}
然后我创建了一个能够做我想要的表达式:
private static System.Linq.Expressions.Expression<Func<String, String, bool>>
Like_Lambda = (item, search) => item.ToLower().Contains(search.ToLower());
private static Func<String, String, bool> Like = Like_Lambda.Compile();
例如
Console.WriteLine(like("McDonalds", "donAld")); // true
Console.WriteLine(like("McDonalds", "King")); // false
但是binaryOpFactory
需要这样:
Func<Expression, Expression, Expression>
预定义的表达式似乎正是这样:
System.Linq.Expressions.Expression.Or;
谁能告诉我如何转换我的表达?
就像是:
static IEnumerable<T> WhereLike<T>(
this IEnumerable<T> data,
string propertyOrFieldName,
string value)
{
var param = Expression.Parameter(typeof(T), "x");
var body = Expression.Call(
typeof(Program).GetMethod("Like",
BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public),
Expression.PropertyOrField(param, propertyOrFieldName),
Expression.Constant(value, typeof(string)));
var lambda = Expression.Lambda<Func<T, bool>>(body, param);
return data.Where(lambda.Compile());
}
static bool Like(string a, string b) {
return a.Contains(b); // just for illustration
}
就Func<Expression,Expression,Expression>
:
static Expression Like(Expression lhs, Expression rhs)
{
return Expression.Call(
typeof(Program).GetMethod("Like",
BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public)
,lhs,rhs);
}
我为IEnumerable
和IQueryable
创建了2个扩展方法WhereFilter()
。 通过这种方式,您可以将此过滤器与例如Entity Framework一起使用,并且是在服务器上执行的过滤。
我使用了基于* (不是?)的过滤器,所以我可以使用底层的Linq方法StartsWith()
, EndsWith()
和Contains()
。 支持的格式:A *,* A,* A *,A * B.
用法:
var filtered = list.WhereFilter(i => i.Name, "a*", "First Name");
这里是班级的基础知识:
/// <summary>
/// Extension Methods for Filtering on IQueryable and IEnumerable
/// </summary>
internal static class WhereFilterExtensions
{
/// <summary>
/// Filters a sequence of values based on a filter with asterix characters: A*, *A, *A*, A*B
/// </summary>
/// <param name="source"></param>
/// <param name="selector">Field to use for filtering. (E.g: item => item.Name)</param>
/// <param name="filter">Filter: A*, *A, *A*, A*B</param>
/// <param name="fieldName">Optional description of filter field used in error messages</param>
/// <returns>Filtered source</returns>
public static IEnumerable<T> WhereFilter<T>(this IEnumerable<T> source, Func<T, string> selector, string filter, string fieldName)
{
if (filter == null)
return source;
if (selector == null)
return source;
int astrixCount = filter.Count(c => c.Equals('*'));
if (astrixCount > 2)
throw new ApplicationException(string.Format("Invalid filter used{0}. '*' can maximum occur 2 times.", fieldName == null ? "" : " for '" + fieldName + "'"));
if (filter.Contains("?"))
throw new ApplicationException(string.Format("Invalid filter used{0}. '?' is not supported, only '*' is supported.", fieldName == null ? "" : " for '" + fieldName + "'"));
// *XX*
if (astrixCount == 2 && filter.Length > 2 && filter.StartsWith("*") && filter.EndsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(item => selector.Invoke(item).Contains(filter));
}
// *XX
if (astrixCount == 1 && filter.Length > 1 && filter.StartsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(item => selector.Invoke(item).EndsWith(filter));
}
// XX*
if (astrixCount == 1 && filter.Length > 1 && filter.EndsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(item => selector.Invoke(item).StartsWith(filter));
}
// X*X
if (astrixCount == 1 && filter.Length > 2 && !filter.StartsWith("*") && !filter.EndsWith("*"))
{
string startsWith = filter.Substring(0, filter.IndexOf('*'));
string endsWith = filter.Substring(filter.IndexOf('*') + 1);
return source.Where(item => selector.Invoke(item).StartsWith(startsWith) && selector.Invoke(item).EndsWith(endsWith));
}
// XX
if (astrixCount == 0 && filter.Length > 0)
{
return source.Where(item => selector.Invoke(item).Equals(filter));
}
// *
if (astrixCount == 1 && filter.Length == 1)
return source;
// Invalid Filter
if (astrixCount > 0)
throw new ApplicationException(string.Format("Invalid filter used{0}.", fieldName == null ? "" : " for '" + fieldName + "'"));
// Empty string: all results
return source;
}
/// <summary>
/// Filters a sequence of values based on a filter with asterix characters: A*, *A, *A*, A*B
/// </summary>
/// <param name="source"></param>
/// <param name="selector">Field to use for filtering. (E.g: item => item.Name) </param>
/// <param name="filter">Filter: A*, *A, *A*, A*B</param>
/// <param name="fieldName">Optional description of filter field used in error messages</param>
/// <returns>Filtered source</returns>
public static IQueryable<T> WhereFilter<T>(this IQueryable<T> source, Expression<Func<T, string>> selector, string filter, string fieldName)
{
if (filter == null)
return source;
if (selector == null)
return source;
int astrixCount = filter.Count(c => c.Equals('*'));
if (astrixCount > 2)
throw new ApplicationException(string.Format("Invalid filter used{0}. '*' can maximum occur 2 times.", fieldName==null?"":" for '" + fieldName + "'"));
if (filter.Contains("?"))
throw new ApplicationException(string.Format("Invalid filter used{0}. '?' is not supported, only '*' is supported.", fieldName == null ? "" : " for '" + fieldName + "'"));
// *XX*
if (astrixCount == 2 && filter.Length > 2 && filter.StartsWith("*") && filter.EndsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(
Expression.Lambda<Func<T, bool>>(
Expression.Call(selector.Body, "Contains", null, Expression.Constant(filter)),
selector.Parameters[0]
)
);
}
// *XX
if (astrixCount == 1 && filter.Length > 1 && filter.StartsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(
Expression.Lambda<Func<T, bool>>(
Expression.Call(selector.Body, "EndsWith", null, Expression.Constant(filter)),
selector.Parameters[0]
)
);
}
// XX*
if (astrixCount == 1 && filter.Length > 1 && filter.EndsWith("*"))
{
filter = filter.Replace("*", "");
return source.Where(
Expression.Lambda<Func<T, bool>>(
Expression.Call(selector.Body, "StartsWith", null, Expression.Constant(filter)),
selector.Parameters[0]
)
);
}
// X*X
if (astrixCount == 1 && filter.Length > 2 && !filter.StartsWith("*") && !filter.EndsWith("*"))
{
string startsWith = filter.Substring(0, filter.IndexOf('*'));
string endsWith = filter.Substring(filter.IndexOf('*') + 1);
return source.Where(
Expression.Lambda<Func<T, bool>>(
Expression.Call(selector.Body, "StartsWith", null, Expression.Constant(startsWith)),
selector.Parameters[0]
)
).Where(
Expression.Lambda<Func<T, bool>>(
Expression.Call(selector.Body, "EndsWith", null, Expression.Constant(endsWith)),
selector.Parameters[0]
)
);
}
// XX
if (astrixCount == 0 && filter.Length > 0)
{
return source.Where(
Expression.Lambda<Func<T, bool>>(
Expression.Equal(selector.Body, Expression.Constant(filter)),
selector.Parameters[0]
)
);
}
// *
if (astrixCount == 1 && filter.Length == 1)
return source;
// Invalid Filter
if (astrixCount > 0)
throw new ApplicationException(string.Format("Invalid filter used{0}.", fieldName == null ? "" : " for '" + fieldName + "'"));
// Empty string: all results
return source;
}
}
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