Using PredicateBuilder to build query searching across multiple columns of Entity

c# entity-framework expression-trees predicatebuilder


I have a list of field names. I am trying to build a predicate to look in the fields to see if they contain the search term. I have gone done the path listed in this original question but do not understand how to do a Contains instead of a NotEqual.

string searchTerm = "Fred";    
foreach (var field in FieldNames)
    myPredicate= myPredicate.And(m => m.*field*.Contains(searchTerm));                    

My code so far:

public static Expression<Func<T, bool>> MultiColumnSearchExpression<T>(string fieldName,string searchValue)
    var parameter = Expression.Parameter(typeof(T), "m");
    var fieldAccess = Expression.PropertyOrField(parameter, fieldName);
   //this next line should do a Contains rather then NotEqual but how?
    var body = Expression.NotEqual(fieldAccess, nullValue);

    var expr = Expression.Lambda<Func<T, bool>>(body, parameter);
    return expr;
5/23/2017 12:10:01 PM

Accepted Answer

So you want to call String.Contains method.

It is a String class instance method with the following signature

public bool Contains(string value)

which can be mapped to the following Expression.Call overload:

public static MethodCallExpression Call(
    Expression instance,
    string methodName,
    Type[] typeArguments,
    params Expression[] arguments

And here is how:

public static Expression<Func<T, bool>> ContainsPredicate<T>(string memberName, string searchValue)
    var parameter = Expression.Parameter(typeof(T), "m");
    var member = Expression.PropertyOrField(parameter, memberName);
    var body = Expression.Call(
        Type.EmptyTypes, // no generic type arguments
    return Expression.Lambda<Func<T, bool>>(body, parameter);
4/28/2017 5:35:22 PM

Popular Answer

Expression.NotEqual is the binary operator !=. Since your Contains call is not a binary operator, there is no direct replacement for this.

Instead, what you need to do is a method call of Contains. So you first need to get the MethodInfo object for the string.Contains function.

It should look like this:

var method = typeof(string).GetMethod("Contains");
var body = Expression.Call(fieldAccess, method, Expression.Constant(searchValue));

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