C# 如何为 Like 创建 System.Linq.Expressions.Expression？
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How to create a System.Linq.Expressions.Expression for Like?
提问by Jürgen Steinblock
I created a filterable BindingList from this source. It works great:
我从这个源创建了一个可过滤的 BindingList 。它工作得很好：
list.Filter("Customer == 'Name'");
does what it should. The internals work like a parser, that converts the expression ==or !=into System.Linq.Expressions.Expression. In this case, ==becomes System.Linq.Expressions.Expression.Equal.
做它应该做的。内部工作就像一个解析器，将表达式==或!=转换为System.Linq.Expressions.Expression. 在这种情况下，==变为System.Linq.Expressions.Expression.Equal。
Unfortunately System.Linq.Expressions.Expressiondoes not contain a like operator and I don't know how to solve this.
不幸的System.Linq.Expressions.Expression是不包含 like 运算符，我不知道如何解决这个问题。
The initial code looks like this:
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);
}
Then I created an expression that will do what I want:
然后我创建了一个可以做我想做的表达式：
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();
e.g.
例如
Console.WriteLine(like("McDonalds", "donAld")); // true
Console.WriteLine(like("McDonalds", "King")); // false
But binaryOpFactoryrequires this:
但binaryOpFactory需要这样：
Func<Expression, Expression, Expression>
The predefined expressions seem to be exactly that:
预定义的表达式似乎正是这样：
System.Linq.Expressions.Expression.Or;
Can anyone tell me how to convert my expression?
谁能告诉我如何转换我的表达？
采纳答案by Marc Gravell
Something like:
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
}
In terms of a Func<Expression,Expression,Expression>:
就 a 而言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);
}
回答by Jo VdB
I created 2 extension methods WhereFilter()for IEnumerableand IQueryable.
At this way you can use this filter also with e.g. Entity Framework and is the filtering performed on the server.
我WhereFilter()为IEnumerable和创建了 2 个扩展方法IQueryable。通过这种方式，您也可以将此过滤器与例如实体框架一起使用，并且是在服务器上执行的过滤。
I used a filter based on *(not ?) so i could use the underlaying Linq methods StartsWith(), EndsWith()and Contains(). Supported formats: A*, *A, *A*, A*B
我使用了一个基于*（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
Usage:
用法：
var filtered = list.WhereFilter(i => i.Name, "a*", "First Name");
Here the basics of the class:
这里是类的基础知识：
/// <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;
}
}
