Using LINQ to evaluate expressions from a string against a datatable

Question

I am trying to write something that will allow custom expressions be stored in strings to be evaluated against various data. After several failed attempts I was able to find something successful, but it doesn't lend itself to the intent.

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Name", typeof(string)));
dt.Columns.Add(new DataColumn("Age", typeof(int)));

dt.Rows.Add(new object[] { "John", 32 });

var test = dt.AsEnumerable().AsQueryable();

// An example of an expression to test
string expression = "Age = 32";

// Failed Attempts
//var result1 = test.Where(expression); // Error received:  {"No property or field 'Age' exists in type 'DataRow'"}
//var result2 = test.Where("it.Field<object>(\"Age\") == 32");  // Error received:  {"No property or field 'Field' exists in type 'DataRow'"}
//var result3 = test.Where("Age=@0", 32);  // Error received:  {"No property or field 'Age' exists in type 'DataRow'"}
//var result4 = test.Where("it[\"Age\"] == 127"); // Error received:  {"Operator '==' incompatible with operand types 'Object' and 'Int32'"}
//var result5 = test.Where("it[\"Age\"] = 127");  // Error received:  {"Operator '=' incompatible with operand types 'Object' and 'Int32'"}
//var result6 = test.Where("it[\"Age\"] as Age  = 127");  // Error received:  {"Expression of type 'Boolean' expected"}

// Successful but at this point its not really a dynamic linq expression
var result = test.Where(it => it.Field<int>("Age") == 32);

How can I accomplish this? All roads seems to lead back to this article: http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx But I do not see the answer to what I am trying to do as he is using strongly typed data.

Thanks!

Answer 1

If you want use DynamicLINQ you need change your code.

1) this expression

// Failed Attempts
string expression = "Age = 32";
var result1 = test.Where(expression); // Error received:  {"No property or field 'Age' exists in type 'DataRow'"}

is same

// Failed Attempts
var result1 = test.Where("Age = 32"); // Error received:  {"No property or field 'Age' exists in type 'DataRow'"}

so DynamicLinq try find property Age for DataRow and since DataRow class don't have this property you get error No property or field 'Age' exists in type 'DataRow'

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2) in this expression

var result2 = test.Where("it.Field<object>(\"Age\") == 32");  // Error received:  {"No property or field 'Field' exists in type 'DataRow'"}

DynamicLinq try find property or method Field, but it is extension method in DataRowExtensions, so you also get error No property or field 'Field' exists in type 'DataRow'

---

3) this expression is same as first with using parameters

var result3 = test.Where("Age=@0", 32);  // Error received:  {"No property or field 'Age' exists in type 'DataRow'"}

---

Next two expression equivalent, becuase when DynamicLinq parse expression, it parse both = and == as Equals

var result4 = test.Where("it[\"Age\"] == 127"); // Error received:  {"Operator '==' incompatible with operand types 'Object' and 'Int32'"}
var result5 = test.Where("it[\"Age\"] = 127");  // Error received:  {"Operator '=' incompatible with operand types 'Object' and 'Int32'"}

if you see about DataRow.Item property you can see that it return Object, so for checking equals you need cast object result to int something like this

var result4 = test.Where("Convert.ToInt32(it[\"Age\"]) == 127");

also, possibly, in this case you can help paramtrize form like this

var result4 = test.Where("it[\"Age\"] == @0",127);

---

at last here

var result6 = test.Where("it[\"Age\"] as Age  = 127");  // Error received:  {"Expression of type 'Boolean' expected"}

after parsing expression DynamicLinq get lambda that don't return Boolean, so you get error Expression of type 'Boolean' expected

Answer 2

This should get you what you're looking for. Put in your null checking in case there are no columns that match the supplied string name etc...

int age = 32;
string column = "Age";

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Name", typeof(string)));
dt.Columns.Add(new DataColumn("Age", typeof(int)));

dt.Rows.Add(new object[] { "John", 32 });

var result = dt.AsEnumerable().Where(r => Convert.ToInt32(r[column]) == age);