Expression Tree To String

      Test objects:

Provides a ToString extension method which returns a string representation of an expression tree (an object inheriting from System.Linq.Expressions.Expression).

Expression<Func<bool>> expr = () => true;

Console.WriteLine(expr.ToString("C#"));
// prints: () => true

Console.WriteLine(expr.ToString("Visual Basic"));
// prints: Function() True

Console.WriteLine(expr.ToString("Factory methods", "C#"));
// prints:
/*
    // using static System.Linq.Expressions.Expression

    Lambda(
        Constant(true)
    )
*/

Console.WriteLine(expr.ToString("Object notation", "C#"));
// prints:
/*
    new Expression<Func<bool>> {
        NodeType = ExpressionType.Lambda,
        Type = typeof(Func<bool>),
        Body = new ConstantExpression {
            Type = typeof(bool),
            Value = true
        },
        ReturnType = typeof(bool)
    }
*/

Console.WriteLine(expr.ToString("Object notation", "Visual Basic"));
// prints:
/*
    New Expression(Of Func(Of Boolean)) With {
        .NodeType = ExpressionType.Lambda,
        .Type = GetType(Func(Of Boolean)),
        .Body = New ConstantExpression With {
            .Type = GetType(Boolean),
            .Value = True
        },
        .ReturnType = GetType(Boolean)
    }
*/

Console.WriteLine(expr.ToString("Textual tree"));
// prints:
/*
    Lambda (Func<bool>)
        Body - Constant (bool) = True
*/

var b = true;
Expression<Func<int>> expr1 = () => b ? 1 : 0;
Console.WriteLine(expr1.ToString("ToString"));
// prints:
/*
    () => IIF(value(_tests.Program+<>c__DisplayClass0_0).b, 1, 0)
*/

Console.WriteLine(expr1.ToString("DebugView"));
// prints:
/*
    .Lambda #Lambda1<System.Func`1[System.Int32]>() {
        .If (
            .Constant<_tests.Program+<>c__DisplayClass0_0>(_tests.Program+<>c__DisplayClass0_0).b
        ) {
            1
        } .Else {
            0
        }
    }
*/

Expression<Func<Person, bool>> filter = p => p => p.LastName == "A" || p.FirstName == "B" || p.DOB == DateTime.MinValue || p.LastName == "C" || p.FirstName == "D";
Console.WriteLine(filter.ToString("Dynamic LINQ", "C#"));
// prints:
/*
    "LastName in (\"A\", \"C\") || FirstName in (\"B\", \"D\") || DOB == DateTime.MinValue"
*/

Features:

• Multiple writers:

  • Pseudo-code in C# or VB.NET
  • Factory method calls which generate this expression
  • Object notation, using object initializer and collection initializer syntax to describe objects
  • Textual tree, focusing on the properties related to the structure of the tree
  • ToString and DebugView reimplementation
  • Dynamic LINQ equivalent to the expression

• For C# and VB pseudo-code representations:

  • Extension methods are rendered as instance methods

    Expression<Func<int, int>> expr = x => Enumerable.Range(1, x).Select(y => x * y).Count();
    Console.WriteLine(expr.ToString("C#"));
    // prints: (int x) => Enumerable.Range(1, x).Select((int y) => x * y).Count()

  • Closed-over variables are rendered as simple identifiers (instead of member access on the hidden compiler-generated class)

    var i = 7;
    var j = 8;
    Expression<Func<int>> expr = () => i + j;
    Console.WriteLine(expr.ToString("C#"));
    // prints: () => i + j

  • Calls to String.Concat and String.Format are mapped to the + operator and string interpolation, respectively (where possible):

    var name = "World";
    Expression<Func<string>> expr = () => string.Format("Hello, {0}!", name);
    Console.WriteLine(expr.ToString("C#"));
    // prints: () => $"Hello, {name}!"

  • Unnecessary conversions are not rendered:

    Expression<Func<IEnumerable<char>>> expr = () => (IEnumerable<char>)"abcd";
    Console.WriteLine(expr.ToString("C#"));
    // prints: () => "abcd"

  • Comparisons against an enum or char are rendered properly, not as comparison to int-converted value:

    var dow = DayOfWeek.Sunday;
    Expression<Func<bool>> expr = () => DateTime.Today.DayOfWeek == dow;
    
    Console.WriteLine(expr.ToString("Textual tree", "C#"));
    // prints:
    /*
      Lambda (Func<bool>)
          · Body - Equal (bool) = false
              · Left - Convert (int) = 3
                  · Operand - MemberAccess (DayOfWeek) DayOfWeek = DayOfWeek.Wednesday
                      · Expression - MemberAccess (DateTime) DateTime.Today = 30/09/2020 12:00:00 am
              · Right - Convert (int) = 0
                  · Operand - MemberAccess (DayOfWeek) dow = DayOfWeek.Sunday
                      · Expression - Constant (<closure>) = #<closure>      
    */
    
    Console.WriteLine(expr.ToString("C#"));
    // prints: () => DateTime.Today.DayOfWeek == dow

• Each representation (including the ToString and DebugView renderers) can return the start and length of the substring corresponding to any of the paths of the tree's nodes, which can be used to find the substring corresponding to a given node in the tree:

  var s = expr.ToString("C#", out var pathSpans);
  Console.WriteLine(s);
  // prints: (Person p) => p.DOB.DayOfWeek == DayOfWeek.Tuesday
  
  (int start, int length) = pathSpans["Body.Left.Operand"];
  Console.WriteLine(s.Substring(start, length));
  // prints: p.DOB.DayOfWeek

• Type names are rendered using language syntax and keywords, instead of the Type.Name property; e.g. List<string> or List(Of Date) instead of List`1

• Supports the full range of types in System.Linq.Expressions, including .NET 4 expression types, and DynamicExpression

  • Expression (and derived types)
  • ElementInit
  • MemberBinding (and derived types)
  • SwitchCase
  • CatchBlock
  • LabelTarget

• Extensibility -- allows creating custom renderers, or inheriting from existing renderers, to handle your own Expression-derived types

For more information, see the wiki.

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