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ModelState. IsValid indicates if it was possible to bind the incoming values from the request to the model correctly and whether any explicitly specified validation rules were broken during the model binding process.
The error typically means that your Model doesn't meet the requirements to be validated. In your example, your FirstName , LastName , Email , Subject and Message properties are decorated with the [Required] attribute. This means that those values shouldn't be null and empty otherwise your condition if(ModelState.
Model state represents errors that come from two subsystems: model binding and model validation. Errors that originate from model binding are generally data conversion errors. For example, an "x" is entered in an integer field.
The ModelState represents a collection of name and value pairs that were submitted to the server during a POST. It also contains a collection of error messages for each value submitted. Despite its name, it doesn't actually know anything about any model classes, it only has names, values, and errors.
ModelState.IsValid tells you if any model errors have been added to ModelState.
The default model binder will add some errors for basic type conversion issues (for example, passing a non-number for something which is an "int"). You can populate ModelState more fully based on whatever validation system you're using.
The sample DataAnnotations model binder will fill model state with validation errors taken from the DataAnnotations attributes on your model.
From the Errata:
ModelState.AddRuleViolations(dinner.GetRuleViolations());
Should be:
ModelState.AddModelErrors(dinner.GetRuleViolations());
Reference: http://www.wrox.com/WileyCDA/WroxTitle/Professional-ASP-NET-MVC-1-0.productCd-0470384611,descCd-ERRATA.html
All the model fields which have definite types, those should be validated when returned to Controller. If any of the model fields are not matching with their defined type, then ModelState.IsValid will return false. Because, These errors will be added in ModelState.
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