CODEWITHSUNDEEP
linqlinq-to-sqllinq-expressions
user1790300
user1790300

Reputation: 1735

LinqExpression: How do I get the value from comparison evaluation

I am trying to translate a LinqExpression into a sql statement. For example, on my Repository base class I have a method Find that accepts a LinqExpression as its argument. Within the Find method, I am calling a method that performs the following action: 

var equality = expression as BinaryExpression;
                return equality.Left.Translate() + " = " +
                       equality.Right.Translate();

How can I get the value of the variable on the right side and not just the variable name, currently I am only getting the variable name. ex. Find(x => x.ID = variable); if variable is currently set to 7 I need a way to get the value 7 and not the name variable. What would be the correct way to get to the value stored in the variable?

Upvotes: 3

Views: 129

Answers (2)

mkoertgen
mkoertgen

Reputation: 982

Try casting into MemberExpression or ConstantExpression respectively:

  var memberExpression = (MemberExpression) expression.Left;
  var constantExpression = (ConstantExpression) expression.Right;

Oh, missed it was a variable. Then the right operand should be of type ParameterException. However, to get its value you may use

 Func<object> getValue = expression.Compile();
 var value = getValue();

Note that this may be slow.

Upvotes: 0

svick
svick

Reputation: 244948

Have a look at Matt Warren's article LINQ: Building an IQueryable Provider - Part III. It contains code for Evaluator class which will inline all values that can be inlined. In your case, it will replace variable with a ConstantExpression that contains the value 7.

The rest of the articles in the series might be of interest to you too.

Upvotes: 1

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