When I use Expression.ToString() to convert an Expression Tree into human readable form, the result is something like this:
x => ((x.ID > 2) OrElse (x.ID != 6))
x => ((x.ID > 2) AndAlso (x.ID != 6))
Ideally, I would want the output to show the operators instead of "OrElse" and "AndAlso":
x => ((x.ID > 2) || (x.ID != 6))
x => ((x.ID > 2) && (x.ID != 6))
As a workaround, I could use the string.Replace() method..
.Replace("AndAlso", "&&")
.Replace("OrElse", "||")
but that has obvious weaknesses and seems awkward.
Also I do not want to create a large 'Replace'-section or huge regex-tree simply to get the formatting right.
Is there a simple way to get a code-like human-readable form of expression trees?
Unfortunately the simplest way to do this correctly would be to write your own ExpressionVisitor class which produces the C#-formatted output code.
The simplest way to do this is to use the ExpressionStringBuilder from the reference sources as a starting point and to tweak it until you are happy with the output result.
When I'm interested in the semantics of the code represented by the expression, rather than the exact syntax tree, I've found it very useful to compile it to an Assembly and view that in ILSpy. Convenience method:
// Code is probably adapted from some other answer, don't remember
public static void CompileToAssemblyFile(
this LambdaExpression expression,
string outputFilePath = null,
string assemblyAndModuleName = null,
string typeName = "TheType",
string methodName = "TheMethod",
// Adjust this
string ilSpyPath = #"C:\path\to\ILSpy.exe")
{
assemblyAndModuleName = assemblyAndModuleName ?? nameof(CompileToAssemblyFile);
outputFilePath = outputFilePath ??
Path.Combine(
Path.GetTempPath(),
$"{assemblyAndModuleName}_{DateTime.Now:yyyy-MM-dd_HH_mm_ss}_{Guid.NewGuid()}.dll");
var domain = AppDomain.CurrentDomain;
var asmName = new AssemblyName {Name = assemblyAndModuleName};
var asmBuilder = domain.DefineDynamicAssembly(
asmName,
AssemblyBuilderAccess.RunAndSave,
Path.GetDirectoryName(outputFilePath));
string outputFileName = Path.GetFileName(outputFilePath);
var module = asmBuilder.DefineDynamicModule(
assemblyAndModuleName,
outputFileName,
true);
var typeBuilder = module.DefineType(typeName, TypeAttributes.Public);
var methodBuilder = typeBuilder.DefineMethod(
methodName,
MethodAttributes.Public | MethodAttributes.Static,
expression.ReturnType,
expression.Parameters.Select(p => p.Type).ToArray());
var pdbGenerator = DebugInfoGenerator.CreatePdbGenerator();
expression.CompileToMethod(methodBuilder, pdbGenerator);
typeBuilder.CreateType();
asmBuilder.Save(outputFileName);
Process.Start(ilSpyPath, outputFilePath);
}
(This is not very faithful to the syntax tree because it goes through both the Expression -> IL translation done by the LambdaCompiler, and the IL -> C# decompilation by ILSpy. OTOH, it can improve readability by converting some gotos into loops, and by producing actual C#.)
This will fail if the Expression contains "non-trivial constants" (live objects); but for that one could write a visitor that replaces constants by new variables and then lambda-abstracts these variables at the toplevel.
Related
I am looking for a way to get a boolean value from a string.
I have a string, stored in a database, that is a condition to be tested with.
suppose the string = "[number] < 10000"
In my code I then plan to replace [number] with a actual number so the string would become "100 < 10000".
Is there a simple way to evaluate this string to a boolean.
In this case it would be False.
Another example: "[number] > 100 && [number] < 1000"
If number would be 200, the evaluation would be True.
What if I add code to this condition, like "if ([number] > 100 && [number] < 1000) { return true;} else {return false;}"
If there a simple way to get the return value from this string with code?
You can make use of DynamicExpression class, it is available in this nuget package. It can evaluate expressions on the fly.
Below is an example,
var expression = System.Linq.Dynamic.DynamicExpression.ParseLambda(
new System.Linq.Expressions.ParameterExpression[] { },
typeof(bool),
"100 > 1000",
new object[] { });
var compiledDelegate = expression.Compile();
var result = compiledDelegate.DynamicInvoke(null);
The result will be false as the provided expression "100 > 1000" is false.
If you need more control, you have to create expressions depending upon dynamic strings representing logical operations. For example, I have taken your first case of "[number]<1000" You can try below:
static void Main(string[] args)
{
string code = "[number] < 10000".Replace("[number]", "100");
string[] operands = code.Split('<');
var comparison = Expression.LessThan(
Expression.Constant(operands[0]),
Expression.Constant(operands[1]));
var lambda = Expression.Lambda<Func<bool>>(comparison).Compile();
bool result = lambda();
}
Or Alternatively you can use existing libraries or nuget packages for doing same thing in a more simpler way (under hood they do same though):
string code = "[number] < 10000".Replace("[number]", "100");
func = ExpressionParser.Compile<Func<bool>>(code);
bool result = func()
It makes use of Lambda parser as shown here
Lets say i have
int a = 1;
int b = 2;
string exp = "b > a";
and i want to evaluate the string expression with those variables
if(exp.SomeKindOfParseOrCast())
{
//here be magic
}
Is it possible in any simple way?
Nope, not in C# - these are parameter names, and thus are compile time values, and this expression parsing you are describing is done in runtime - the computer doesn't know the name of the parameters while it's being evaluated. Instead, you could do something a little more strict, like an expression parser - implement your own way to parse string expressions.
Very very simplified:
if(exp.Equals("b > a"))
{
if(b>a)
// do what you do if b is bigger than a
else
// do what you do with a wrong expression
}
else if (exp.Equals("a > b")
{
if(a>b)
// do what you do if a is bigger than b
else
// do what you do with a wrong expression
}
else if (exp.Equals("a = b")
{
if(a==b)
// do what you do if a is equal to b
else
// do what you do with a wrong expression
}
else
// do what you do with a badly formatted expression
if you would like to take this a step forward, you can cut spaces, make sure the expression is lowercase, etc. - there's many examples around, I personally like this one.
Is it possible in any simple way?
No, in C# this is not possible in a simple way like it were in languages such as JavaScript with its eval function. Anyway, you'd have to provide bindings of in-expression parameters to actual values.
You can use Roslyn.
Here is an example of how to compile and run your own code in runtime.
Disclaimer: I'm the owner of the project Eval Expression.NET
This library is very easy to use and allow to evaluate and compile almost all the C# language.
// For single evaluation
var value1 = Eval.Execute<bool>("b > a", new { a = 1, b = 2 });
// For many evaluation
var compiled = Eval.Compile<Func<int, int, bool>>("b > a", "a", "b");
var value2 = compiled(1, 2);
let assume we have an array like this:
var caps = new[] { "1512x", "001xx", "27058", "201xx", "4756x" };
(original array is huge and come from another linq query)
What I need is to create a LINQ statement accepting a value and tries to match with one of the values on the foreseen array.
For example if I use "15121" I need to match the "1512x" value on the array and return it.
Obviously, if I use "27058" it finds the exact match and simply return it.
Is it possible in LINQ?
The "wildcard" char on the array is "x", but I can change it.
Thanks in advance!
Valerio
You can use regular expressions:
var value = "15121";
var caps = new[] { "1512x", "001xx", "27058", "201xx", "4756x" };
var match = caps
.FirstOrDefault(c => new Regex("^" + c.Replace("x", "[0-9]") + "$").IsMatch(value));
if (match != null)
Console.WriteLine("{0} matches {1}", value, match);
The "pattern" 001xx is converted into the regular expression ^001[0-9][0-9]$ and so on. Then the first matching regular expressions is found.
But if the caps is huge it might not perform so well because each regular expression has to be compiled and converted into a state machine until a match is found.
Assuming you have a predicate method, something like this (or something equivalent using Regex, as described in another answer):
static bool Match(string pattern, string exact)
{
if(pattern.Length != exact.Length) return false;
for(var i = 0; i < pattern.Length; i++)
if(pattern[i] != exact[i] && pattern[i] != 'x')
return false;
return true;
}
Then the LINQ query can look like this:
var found = caps.Single(x => Match(x, yourSearch));
I have a scenario in which I am saving my "if" conditions in database as a string. For example:
String condition = "(([age] >= 28) && ([nationality] == 'US'))";
OR
String condition = "([age] >= 28)";
Now, I want to evaluate that the user has input the condition syntactically correct. These are example of incorrect syntax:
String condition = "(([age] >= 28) && ([nationality] == 'US')"; //Missed ')' bracket
String condition = "[age] >= 28)"; //Missed Opening bracket '('
Like we have in Evaluate Query Expression. Might be Expression tress can be helpful. But how? Need help in this regard.
Take a look at NCalc. It's a framework for evaluating mathematical expressions.
When the expression has a syntax error, the evaluation will throw an EvaluationException.
try
{
new Expression("(3 + 2").Evaluate();
}
catch(EvaluationException e)
{
Console.WriteLine("Error catched: " + e.Message);
}
Though, you can also detect syntax errors before the evaluation by using the HasErrors() method.
Expression e = new Expression("a + b * (");
if(e.HasErrors())
{
Console.WriteLine(e.Error);
}
Visual studio doesn't really know what the strings represent so to my knowledge there is no parsing done within the strings themselves.
Typically when programming with C# and using sql, you'd try to do as much of the calculations as possible in C# itself (if it's feasible select the whole table then deal with the result using C#).
If the database is really slow which is quite often the case, it may be useful writing a SQL Builder class to deal with the hardcoded strings.
If you use neither of these methods, unfortunately the best you can really hope for is runtime exceptions (which isn't optimal for obvious reasons).
EDIT:
It seems a SelectQueryBuilder library already exists for the second scenario I suggested.
I found this solution
evaluate an arithmetic expression stored in a string (C#)
SOLUTION:
string conditiontext = "(([age] >= 28) && ([nationality] == \"US\"))";
conditiontext = conditiontext.Replace("[age]", 32)
.Replace("[nationality]","US");
/*VsaEngine*/
var engine = Microsoft.JScript.Vsa.VsaEngine.CreateEngine();
/** Result will be either true or false based on evaluation string*/
var result = Microsoft.JScript.Eval.JScriptEvaluate(conditiontext, engine);
[Note: This interface is deprecated. But it evaluates any arithmetic expressions and c# expressions]
You could use System.Data and its DataTable.Compute() method.
Here is the code:
public bool CheckCondition()
{
// parameters
(string name, object value)[] variables =new (string name, object value)[1];
variables[0].name = "age";
variables[0].value = 28;
variables[1].name = "nationality";
variables[1].value = "US";
string conditions = "(([age] >= 28) && ([nationality] == 'US'))";
conditions.Replace("[", "").Replace("]", "").Replace("&&", "AND").Replace("||", "OR");
using DataTable table = new DataTable();
foreach (var (name, value) in variables)
table.Columns.Add(name, value is null ? typeof(object) : value.GetType());
table.Rows.Add();
foreach (var (name, value) in variables)
table.Rows[0][name] = value;
table.Columns.Add("_Result", typeof(double)).Expression = conditions
?? throw new ArgumentNullException(nameof(conditions));
return (bool)(Convert.ChangeType(table.Compute($"Min(_Result)", null), typeof(bool)));
}
Basically I have some code where when it happens, I need to set some object equal to some expression. All of this "what to do" jazz is stored as a string. So I parse it, and use reflection to find the object I am doing it to. Now I need to find out how to store the value to this object. The problem is the value could be "1", "1*(5/2)", or "some string value". It would be really cool if I could have expressions like "this.SomeProperty" or "(x > 3 ? 4 : 5)".
Also, the object it is storing to, could be a string, int, double, or float at the minimum.
The VS2008 samples included a nifty ExpressionParser which could be used as a generic expression parser (VS2008 Samples). With a few small updates, and a custom factory class, we can turn it into something a bit more expressive:
string expression = "(1 + 2)";
var func = FunctionFactory.Create<int>(expression);
Or:
expression = "(a * b)";
var func2 = FunctionFactory.Create<int, int, int>(expression, new[] { "a", "b" });
The return types of these Create methods are Func<> instances, which means we get nice strongly type delegates:
int result = func2(45, 100); // result = 450;
I've push the code to a gist
Update: I've recently blogged about this too.
Update 2, another example:
var person = new Person { Age = 5 };
string expression = "(Age == 5)";
var func3 = FunctionFactory.Create<Person, bool>(expression);
bool isFive = func3(person); // Should be true.
Have you seen http://ncalc.codeplex.com ?
It's extensible, fast (e.g. has its own cache) enables you to provide custom functions and varaibles at run time by handling EvaluateFunction/EvaluateParameter events. Example expressions it can parse:
Expression e = new Expression("Round(Pow(Pi, 2) + Pow([Pi2], 2) + X, 2)");
e.Parameters["Pi2"] = new Expression("Pi * Pi");
e.Parameters["X"] = 10;
e.EvaluateParameter += delegate(string name, ParameterArgs args)
{
if (name == "Pi")
args.Result = 3.14;
};
Debug.Assert(117.07 == e.Evaluate());
It also handles unicode & many data type natively. It comes with an antler file if you want to change the grammer. There is also a fork which supports MEF to load new functions.