As part of a project that we are working on at the moment\u00a0 we need a way of converting a literal or constant into a lambda expression which when evaluated will\u00a0yield the original literal value (in c#).\u00a0 This must work in the same way for multiple types.<\/p>\n
\u00a0<\/p>\n
For example given the literal string “hello” we need to programmatically create the lambda<\/p>\n
\u00a0<\/p>\n
() => “hello”<\/p>\n
\u00a0<\/p>\n
and for the boolean true we want to create:<\/p>\n
\u00a0<\/p>\n
() => true<\/p>\n
\u00a0<\/p>\n
etc.<\/p>\n
\u00a0<\/p>\n
This can be achieved by creating an appropriate expression tree and then compiling it into a lambda expression, and as we want this to work for different types we wrap all of this into a generic function like the following:<\/p>\n
\u00a0<\/p>\n
\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 static public Func<T> ToLambda<T>(T val)
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ConstantExpression body = Expression.Constant(val);
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 LambdaExpression exp = Expression.Lambda<Func<T>>(body, null);<\/p>\n
\u00a0<\/p>\n
\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return (Func<T>)exp.Compile();
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }<\/p>\n
\u00a0<\/p>\n
The function takes the value in question as type T and returns a\u00a0parameter-less delegate of return\u00a0type T.\u00a0 It\u00a0uses the passed value to create a ConstantExpression,\u00a0and then uses it to create a LambdaExpression, as the required lambda has no paramters it passes ‘null’ into the constructor’s second parameter.\u00a0 Now all that is left is to compile the LambdaExpression, this creates and returns the required delegate which is then returned.<\/p>\n
\u00a0<\/p>\n
The following code shows the\u00a0lambda generator\u00a0in action:<\/p>\n
\u00a0<\/p>\n
\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 static public void TestLiteralToLambda()
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \/\/ Create the Lambda from literal
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 var boolLambda = ToLambda(true);
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \/\/ Evaluate the lambda..
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 bool boolVal = boolLambda();
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \/\/ and, log out some info
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Console.WriteLine(“Lambda evaluates to – {0}”, boolVal);<\/p>\n
\u00a0<\/p>\n
\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 var stringLambda = ToLambda(“Rainy day in Dublin”);
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 string stringVal = stringLambda();
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Console.WriteLine(“Lambda evaluates to – {0}”, stringVal);<\/p>\n
\u00a0<\/p>\n
\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 var doubleLambda = ToLambda(3.14159);
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 double doubleVal = doubleLambda();
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Console.WriteLine(“Lambda evaluates to – {0}”, doubleVal);\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0?
\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }<\/p>\n
\u00a0<\/p>\n
So far, so good.\u00a0 Although this might seem a little strange and vaguely useless, what it allows us to do is to treat literals and lambdas of the same type in exactly the same way.<\/p>\n
\u00a0<\/p>\n
Specifically what we want to be able to do is to define ‘super properties’ on objects whose values when queried are either regular values (as normal) or are the result of evaluating a rule, this rule being provided as a lambda expression.\u00a0 When the property’s value is set the programmer either provides a value (literal or constant) or a rule (lambda expression).<\/p>\n
\u00a0<\/p>\n
Further details\u00a0on the exact makeup of\u00a0these ‘super properties’ elude us at the moment, more later…<\/p>\n","protected":false},"excerpt":{"rendered":"
As part of a project that we are working on at the moment\u00a0 we need a way of converting a literal or constant into a lambda expression which when evaluated will\u00a0yield the original literal value (in c#).\u00a0 This must work in the same way for multiple types. \u00a0 For example given the literal string “hello” […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[11,12],"class_list":["post-39","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-cplusplus","tag-lambda-expression"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t