c# - can I have a nested lambda expression stored in a Dictionary? -


i'd parse string lambda expression in c#, example, parse "lt 5" x => x < 5 can use argument of enumerable.where:

static void main(string[] args) {     enumerable         .range(0,10)         .select(x => (double)x)         .where(parse("lt 5"))         .tolist()         .foreach(system.console.writeline); }  private const list<tuple<list<string>, func<double, func<double, bool>>>> operationlist =     new dictionary<string, func<double, func<double, bool>>>()     { { "lt <",     val => ( x => x <  val ) }     , { "le <=",    val => ( x => x <= val ) }     , { "eq = ==" , val => ( x => x == val ) }     , { "ne != <>", val => ( x => x != val ) }     , { "ge >=",    val => ( x => x >= val ) }     , { "gt >",     val => ( x => x >  val ) } }     .select(kv =>         new tuple<             list<string>,             func<double, func<double, bool>>>(                 kv.key.split(' ').tolist(),                 kv.value))     .tolist();  public static func<double, bool> parse(string raw) {     var fields = raw.split(' ');     var predkey = fields[0].tolower();      var predbuilder = operationlist.firstordefault(tp =>         tp.item1.firstordefault(key => key.equals(predkey)) != null);      return predbuilder.item2(double.parse(fields[1])); }

first made operationlist stores tuple item1 describes aliases operator , item2 stores curried version of operator.

then parse split string lt 5 ["lt","5"] , search operation according key lt.

finally, if can find operation, 5 partial applied , result lambda expression x => x < val.

however, compiler mad @ { "lt <", val => ( x => x < val ) } , complained "expression cannot contain anonymous methods or lambda expressions".

i have no idea means , how things work.

change const readonly , make static, so:

private readonly static list<tuple<list<string>, func<double, func<double, bool>>>> operationlist =     new dictionary<string, func<double, func<double, bool>>>()     { { "lt <",     val => ( x => x <  val ) }     , { "le <=",    val => ( x => x <= val ) }     , { "eq = ==" , val => ( x => x == val ) }     , { "ne != <>", val => ( x => x != val ) }     , { "ge >=",    val => ( x => x >= val ) }     , { "gt >",     val => ( x => x >  val ) } }     .select(kv =>         new tuple<             list<string>,             func<double, func<double, bool>>>(                 kv.key.split(' ').tolist(),                 kv.value))     .tolist();

in c# const keyword used denote compile time constants, in example list calculated @ runtime using linq.

in c# const values static , making readonly change field's scope instance.

note, contents of list might change during execution , readonly won't stop removing , adding tuples list. consider using readonlycollection (it's still won't stop modifying it, make harder) or using immutable collections (which aren't part of .net bcl)


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