How to dynamically create an Expression<Func<MyClass, bool>> predicate from Expression<Func<MyClass, string>> in C#?

To dynamically create an Expression<Func<MyClass, bool>> predicate from an Expression<Func<MyClass, string>>, you'll need to use the Expression class provided by the System.Linq.Expressions namespace. The idea is to convert the original string expression into a bool expression by adding some logical conditions. Below is a step-by-step guide on how to achieve this:

Let's say you have the following class:

public class MyClass { public string Name { get; set; } } 

And you have an existing Expression<Func<MyClass, string>> representing a selector that retrieves the Name property. For example:

Expression<Func<MyClass, string>> nameSelector = mc => mc.Name; 

Now, you want to dynamically create a predicate to check if the name starts with a certain substring, let's say "John". The resulting predicate should be of type Expression<Func<MyClass, bool>>. Here's how you can achieve this:

using System; using System.Linq.Expressions; public class MyClass { public string Name { get; set; } } class Program { static void Main() { // Existing selector expression Expression<Func<MyClass, string>> nameSelector = mc => mc.Name; // Target substring to check string targetSubstring = "John"; // Create the predicate expression Expression<Func<MyClass, bool>> predicate = CreatePredicate(nameSelector, targetSubstring); // Compile and test the predicate var testInstance = new MyClass { Name = "John Doe" }; var compiledPredicate = predicate.Compile(); bool result = compiledPredicate(testInstance); Console.WriteLine(result); // Output: True } static Expression<Func<MyClass, bool>> CreatePredicate(Expression<Func<MyClass, string>> selector, string targetSubstring) { // Parameter expression (input parameter for MyClass) ParameterExpression parameter = selector.Parameters[0]; // Access the property from the original selector MemberExpression propertyAccess = selector.Body as MemberExpression; // Create the left side of the binary expression (selector) Expression left = propertyAccess ?? selector.Body; // Create the right side of the binary expression (target substring) Expression right = Expression.Constant(targetSubstring); // Create the comparison expression (StartsWith) MethodInfo startsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) }); Expression condition = Expression.Call(left, startsWithMethod, right); // Create the lambda expression with the condition Expression<Func<MyClass, bool>> predicate = Expression.Lambda<Func<MyClass, bool>>(condition, parameter); return predicate; } } 

Explanation:

1. The CreatePredicate method takes the original Expression<Func<MyClass, string>> selector and the targetSubstring as inputs.

2. The method first extracts the ParameterExpression representing the input parameter (MyClass) from the original selector.

3. It then accesses the property expression (e.g., mc.Name) from the original selector. If the original selector is more complex (e.g., mc => mc.SomeOtherProperty.Name), it still correctly retrieves the property expression.

4. It creates a constant expression with the targetSubstring.

5. Using the MethodInfo of the string.StartsWith method, it creates a method call expression representing the condition left.StartsWith(right).

6. Finally, it creates a new Expression<Func<MyClass, bool>> lambda expression with the condition and the original parameter.

The resulting predicate will be of type Expression<Func<MyClass, bool>>, representing the condition mc => mc.Name.StartsWith("John"). When compiled and executed, this predicate will return true for any instance of MyClass with a Name property that starts with "John".

Examples

1. "C# dynamically convert Expression<Func<MyClass, string>> to Expression<Func<MyClass, bool>>"

   • Description: This query is about dynamically creating an Expression<Func<MyClass, bool>> predicate from an existing Expression<Func<MyClass, string>>.

   // Example: Dynamically converting Expression<Func<MyClass, string>> to Expression<Func<MyClass, bool>> Expression<Func<MyClass, string>> stringExpression = // Your existing string expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.Equal(stringExpression.Body, Expression.Constant("TargetValue")), stringExpression.Parameters); 

2. "C# dynamically create Expression<Func<MyClass, bool>> from Expression<Func<MyClass, T>>"

   • Description: This query focuses on dynamically creating an Expression<Func<MyClass, bool>> from a generic Expression<Func<MyClass, T>>.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> from Expression<Func<MyClass, T>> Expression<Func<MyClass, T>> genericExpression = // Your existing generic expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.Equal(genericExpression.Body, Expression.Constant(default(T))), genericExpression.Parameters); 

3. "C# dynamically create Expression<Func<MyClass, bool>> with custom condition"

   • Description: This query looks for examples of dynamically creating an Expression<Func<MyClass, bool>> with a custom condition from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with custom condition Expression<Func<MyClass, string>> stringExpression = // Your existing string expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.NotEqual(stringExpression.Body, Expression.Constant("InvalidValue")), stringExpression.Parameters); 

4. "C# dynamically create Expression<Func<MyClass, bool>> with method call"

   • Description: Searching for examples of dynamically creating an Expression<Func<MyClass, bool>> with a method call from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with method call Expression<Func<MyClass, string>> stringExpression = // Your existing string expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.Call(stringExpression.Body, typeof(string).GetMethod("Contains"), Expression.Constant("Substring")), stringExpression.Parameters); 

5. "C# dynamically create Expression<Func<MyClass, bool>> with multiple conditions"

   • Description: This query is about dynamically creating an Expression<Func<MyClass, bool>> with multiple conditions from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with multiple conditions Expression<Func<MyClass, string>> stringExpression1 = // Your existing string expression 1 Expression<Func<MyClass, string>> stringExpression2 = // Your existing string expression 2 Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.AndAlso( Expression.NotEqual(stringExpression1.Body, Expression.Constant("InvalidValue1")), Expression.NotEqual(stringExpression2.Body, Expression.Constant("InvalidValue2")) ), stringExpression1.Parameters); 

6. "C# dynamically create Expression<Func<MyClass, bool>> with null check"

   • Description: Searching for examples of dynamically creating an Expression<Func<MyClass, bool>> with a null check from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with null check Expression<Func<MyClass, string>> stringExpression = // Your existing string expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.NotEqual(stringExpression.Body, Expression.Constant(null, typeof(string))), stringExpression.Parameters); 

7. "C# dynamically create Expression<Func<MyClass, bool>> with custom conversion"

   • Description: This query focuses on dynamically creating an Expression<Func<MyClass, bool>> with a custom type conversion from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with custom conversion Expression<Func<MyClass, int>> intExpression = // Your existing int expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.GreaterThan(intExpression.Body, Expression.Constant(10)), intExpression.Parameters); 

8. "C# dynamically create Expression<Func<MyClass, bool>> with dynamic parameter"

   • Description: Searching for examples of dynamically creating an Expression<Func<MyClass, bool>> with a dynamically determined parameter from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with dynamic parameter ParameterExpression dynamicParameter = Expression.Parameter(typeof(MyClass), "x"); Expression<Func<MyClass, string>> stringExpression = // Your existing string expression Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.NotEqual(Expression.PropertyOrField(dynamicParameter, "Property1"), Expression.Constant("InvalidValue")), dynamicParameter); 

9. "C# dynamically create Expression<Func<MyClass, bool>> with OR condition"

   • Description: This query looks for examples of dynamically creating an Expression<Func<MyClass, bool>> with an OR condition from an existing expression.

   // Example: Dynamically creating Expression<Func<MyClass, bool>> with OR condition Expression<Func<MyClass, string>> stringExpression1 = // Your existing string expression 1 Expression<Func<MyClass, string>> stringExpression2 = // Your existing string expression 2 Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.OrElse( Expression.Equal(stringExpression1.Body, Expression.Constant("ValidValue1")), Expression.Equal(stringExpression2.Body, Expression.Constant("ValidValue2")) ), stringExpression1.Parameters); 

10. "C# dynamically create Expression<Func<MyClass, bool>> with custom logic"

    • Description: Searching for examples of dynamically creating an Expression<Func<MyClass, bool>> with custom logic or complex conditions from an existing expression.

    // Example: Dynamically creating Expression<Func<MyClass, bool>> with custom logic Expression<Func<MyClass, string>> stringExpression1 = // Your existing string expression 1 Expression<Func<MyClass, string>> stringExpression2 = // Your existing string expression 2 Expression<Func<MyClass, bool>> boolExpression = Expression.Lambda<Func<MyClass, bool>>( Expression.AndAlso( Expression.NotEqual(stringExpression1.Body, Expression.Constant("InvalidValue1")), Expression.OrElse( Expression.Equal(stringExpression2.Body, Expression.Constant("ValidValue2")), Expression.Equal(stringExpression2.Body, Expression.Constant("ValidValue3")) ) ), stringExpression1.Parameters); 

---

More Tags

placeholder C++ export wmi gmail-api turi-create android-proguard ord azure-resource-manager sql-drop

More C# Questions

• C# Datagridview how to sort Checkbox column
• How to change time in DateTime using C#?
• How to check for file lock in C#?
• What does it mean for a property to be [Required] and nullable in C#?
• Validating Recaptcha 2 (No CAPTCHA reCAPTCHA) in ASP.NET's server side
• Xamarin Forms File Provider not set
• Upload file to Google Drive with C#
• How to allow implicit conversion in C#?

More General chemistry Calculators

• Free Online Electronegativity Calculator
• Free Online Electron Configuration Calculator
• Free Online Effective Nuclear Charge Calculator
• Free Online Molar Mass Calculator
• Free Online Average Atomic Mass Calculator

More Chemistry Calculators

• Free Online Beer-Lambert Law Calculator
• Free Online Cubic Cell Calculator
• Free Online Detention Time Calculator
• Free Online Hydraulic Retention Time Calculator
• Free Online Miller Indices Calculator

More Fitness Calculators

• Free Online BMI Calculator
• Free Online BMR Calculator
• Free Online Healthy Weight Calculator
• Free Online Ideal Weight Calculator
• Free Online Body Fat Calculator

More Entertainment Anecdotes Calculators

• Free Online Virtual Dice Roller
• Free Online Love Calculator